Reset blend string for Shell.GLSLEffect

GLSLEffect change default blend string from Cogl.OffscreenEffect, it
will make window become grey when try to change it's opacity.

Also this commit can fix #67, when using gnome-screenshots or builtin
tools to teke screenshots, it will not have transparent part.

src/effect/rounded_corners_effect.ts

@@ -8,8 +8,9 @@ import * as types                  from '@me/utils/types'
 
 // types
 import { Me }                      from '@global'
+import { PaintContext, PaintNode } from '@gi/Clutter'
 
-// -------------------------------------------------------------- [end imports]
+// --------------------------------------------------------------- [end imports]
 
 // Load fragment shader of rounded corners effect.
 const { declarations, code } = loadShader (
@@ -65,6 +66,14 @@ export const RoundedCornersEffect = registerClass (
       this._init_uniforms ()
     }
 
+    vfunc_paint_target (node: PaintNode, ctx: PaintContext) {
+      // Reset to default blend string.
+      this.get_pipeline ()?.set_blend (
+        'RGBA = ADD(SRC_COLOR, DST_COLOR*(1-SRC_COLOR[A]))'
+      )
+      super.vfunc_paint_target (node, ctx)
+    }
+
     /**
      * Used to update uniform variants of shader
      * @param corners_cfg   - Rounded corners settings of window

src/effect/shader/rounded_corners.frag

@@ -97,20 +97,13 @@ void main() {
   float outer_alpha = rounded_rect_coverage(texture_coord, bounds, radius, exponent);
 
   // Clip window corners first
-  cogl_color_out.a *= outer_alpha;
+  cogl_color_out *= outer_alpha;
 
   // Draw border later
   if(border_width > 0.9 || border_width < -0.9) {
     float inner_alpha = rounded_rect_coverage(texture_coord, inner_bounds, inner_radius, exponent);
     float border_alpha = clamp(abs(outer_alpha - inner_alpha), 0.0, 1.0);
 
-    if (border_width > 0.9) {
-      // If border is drawing inside window, mix color of window
-      cogl_color_out = mix(cogl_color_out, vec4(border_color.rgb, 1.0), border_alpha * border_color.a);
-      cogl_color_out.a *= outer_alpha;
-    } else {
-      // If border is drawing outside window, just draw border
-      cogl_color_out = mix(cogl_color_out, border_color, border_alpha);
-    }
+    cogl_color_out = mix(cogl_color_out, vec4(border_color.rgb, 1.0), border_alpha * border_color.a);
   }
 }