Change vsync timing to match CEA-861-D

The timing diagrams show that vsync pulses begin and end at the
leading edge of hsync.  Match this behavior.

Also rename some internal signals to use the right nomenclature.
The porches are the periods during blanking before and after
the sync pulses.

src/hdmi.sv

@@ -94,8 +94,8 @@ localparam int NUM_CHANNELS = 3;
 logic hsync;
 logic vsync;
 
-logic [BIT_WIDTH-1:0] hsync_porch_start, hsync_porch_size;
-logic [BIT_HEIGHT-1:0] vsync_porch_start, vsync_porch_size;
+logic [BIT_WIDTH-1:0] hsync_pulse_start, hsync_pulse_size;
+logic [BIT_HEIGHT-1:0] vsync_pulse_start, vsync_pulse_size;
 logic invert;
 
 // See CEA-861-D for more specifics formats described below.
@@ -107,10 +107,10 @@ generate
             assign frame_height = 525;
             assign screen_width = 640;
             assign screen_height = 480;
-            assign hsync_porch_start = 16;
-            assign hsync_porch_size = 96;
-            assign vsync_porch_start = 10;
-            assign vsync_porch_size = 2;
+            assign hsync_pulse_start = 16;
+            assign hsync_pulse_size = 96;
+            assign vsync_pulse_start = 10;
+            assign vsync_pulse_size = 2;
             assign invert = 1;
             end
         2, 3:
@@ -119,10 +119,10 @@ generate
             assign frame_height = 525;
             assign screen_width = 720;
             assign screen_height = 480;
-            assign hsync_porch_start = 16;
-            assign hsync_porch_size = 62;
-            assign vsync_porch_start = 9;
-            assign vsync_porch_size = 6;
+            assign hsync_pulse_start = 16;
+            assign hsync_pulse_size = 62;
+            assign vsync_pulse_start = 9;
+            assign vsync_pulse_size = 6;
             assign invert = 1;
             end
         4:
@@ -131,10 +131,10 @@ generate
             assign frame_height = 750;
             assign screen_width = 1280;
             assign screen_height = 720;
-            assign hsync_porch_start = 110;
-            assign hsync_porch_size = 40;
-            assign vsync_porch_start = 5;
-            assign vsync_porch_size = 5;
+            assign hsync_pulse_start = 110;
+            assign hsync_pulse_size = 40;
+            assign vsync_pulse_start = 5;
+            assign vsync_pulse_size = 5;
             assign invert = 0;
         end
         16, 34:
@@ -143,10 +143,10 @@ generate
             assign frame_height = 1125;
             assign screen_width = 1920;
             assign screen_height = 1080;
-            assign hsync_porch_start = 88;
-            assign hsync_porch_size = 44;
-            assign vsync_porch_start = 4;
-            assign vsync_porch_size = 5;
+            assign hsync_pulse_start = 88;
+            assign hsync_pulse_size = 44;
+            assign vsync_pulse_start = 4;
+            assign vsync_pulse_size = 5;
             assign invert = 0;
         end
         17, 18:
@@ -155,10 +155,10 @@ generate
             assign frame_height = 625;
             assign screen_width = 720;
             assign screen_height = 576;
-            assign hsync_porch_start = 12;
-            assign hsync_porch_size = 64;
-            assign vsync_porch_start = 5;
-            assign vsync_porch_size = 5;
+            assign hsync_pulse_start = 12;
+            assign hsync_pulse_size = 64;
+            assign vsync_pulse_start = 5;
+            assign vsync_pulse_size = 5;
             assign invert = 1;
         end
         19:
@@ -167,10 +167,10 @@ generate
             assign frame_height = 750;
             assign screen_width = 1280;
             assign screen_height = 720;
-            assign hsync_porch_start = 440;
-            assign hsync_porch_size = 40;
-            assign vsync_porch_start = 5;
-            assign vsync_porch_size = 5;
+            assign hsync_pulse_start = 440;
+            assign hsync_pulse_size = 40;
+            assign vsync_pulse_start = 5;
+            assign vsync_pulse_size = 5;
             assign invert = 0;
         end
         95, 105, 97, 107:
@@ -179,17 +179,27 @@ generate
             assign frame_height = 2250;
             assign screen_width = 3840;
             assign screen_height = 2160;
-            assign hsync_porch_start = 176;
-            assign hsync_porch_size = 88;
-            assign vsync_porch_start = 8;
-            assign vsync_porch_size = 10;
+            assign hsync_pulse_start = 176;
+            assign hsync_pulse_size = 88;
+            assign vsync_pulse_start = 8;
+            assign vsync_pulse_size = 10;
             assign invert = 0;
         end
     endcase
-    assign hsync = invert ^ (cx >= screen_width + hsync_porch_start && cx < screen_width + hsync_porch_start + hsync_porch_size);
-    assign vsync = invert ^ (cy >= screen_height + vsync_porch_start && cy < screen_height + vsync_porch_start + vsync_porch_size);
 endgenerate
 
+always_comb begin
+    hsync <= invert ^ (cx >= screen_width + hsync_pulse_start && cx < screen_width + hsync_pulse_start + hsync_pulse_size);
+    // vsync pulses should begin and end at the start of hsync, so special
+    // handling is required for the lines on which vsync starts and ends
+    if (cy == screen_height + vsync_pulse_start)
+        vsync <= invert ^ (cx >= screen_width + hsync_pulse_start);
+    else if (cy == screen_height + vsync_pulse_start + vsync_pulse_size)
+        vsync <= invert ^ (cx < screen_width + hsync_pulse_start);
+    else
+        vsync <= invert ^ (cy >= screen_height + vsync_pulse_start && cy < screen_height + vsync_pulse_start + vsync_pulse_size);
+end
+
 localparam real VIDEO_RATE = (VIDEO_ID_CODE == 1 ? 25.2E6
     : VIDEO_ID_CODE == 2 || VIDEO_ID_CODE == 3 ? 27.027E6
     : VIDEO_ID_CODE == 4 ? 74.25E6