Fix currentcolor detection when parsing gradient color stops

The bug has been introduced by CSS Color Level 4, that set the official
"currentColor"’s serialization string to lowercase "currentcolor".

Note that we still don’t draw the actual current color in this case (according
to the TODO above the fixed line) but black. At least it doesn’t crash
anymore.

Fix #2320.

tests/draw/test_gradient.py

@@ -278,3 +278,15 @@ def test_radial_gradients_5(assert_pixels):
         SzzzzzzzzS
     ''', '''<style>@page { size: 10px 16px; background:
       radial-gradient(red 50%, rgba(255, 0, 0, 0.751) 50%)''')
+
+
+@assert_no_logs
+def test_linear_gradients_currentcolor(assert_pixels):
+    # Test regression: https://github.com/Kozea/WeasyPrint/issues/1561
+    assert_pixels('''
+        KKKKK
+        KKKKK
+        KKKKK
+        KKKKK
+        KKKKK
+    ''', '<style>@page { size: 5px 5px; background: linear-gradient(currentcolor)')

weasyprint/css/utils.py

@@ -386,7 +386,7 @@ def parse_radial_gradient_parameters(arguments):
 def parse_color_stop(tokens):
     if len(tokens) == 1:
         color = parse_color(tokens[0])
-        if color == 'currentColor':
+        if color == 'currentcolor':
             # TODO: return the current color instead
             return parse_color('black'), None
         if color is not None: