gl: Interpolate gradient colors as premultiplied

Tests included

gsk/gl/resources/conic_gradient.glsl

@@ -75,10 +75,10 @@ void main() {
 
     if (offset < next_offset) {
       float f = (offset - curr_offset) / (next_offset - curr_offset);
-      vec4 curr_color = get_color(i);
-      vec4 next_color = get_color(i + 1);
+      vec4 curr_color = gsk_scaled_premultiply (get_color(i), u_alpha);
+      vec4 next_color = gsk_scaled_premultiply (get_color(i + 1), u_alpha);
       vec4 color = mix(curr_color, next_color, f);
-      gskSetScaledOutputColor(gsk_premultiply(color), u_alpha);
+      gskSetOutputColor(color);
       return;
     }
   }

gsk/gl/resources/linear_gradient.glsl

@@ -97,10 +97,10 @@ void main() {
 
     if (offset < next_offset) {
       float f = (offset - curr_offset) / (next_offset - curr_offset);
-      vec4 curr_color = get_color(i);
-      vec4 next_color = get_color(i + 1);
+      vec4 curr_color = gsk_scaled_premultiply (get_color(i), u_alpha);
+      vec4 next_color = gsk_scaled_premultiply (get_color(i + 1), u_alpha);
       vec4 color = mix(curr_color, next_color, f);
-      gskSetScaledOutputColor(gsk_premultiply (color), u_alpha);
+      gskSetOutputColor(color);
       return;
     }
   }

gsk/gl/resources/radial_gradient.glsl

@@ -77,10 +77,10 @@ void main() {
 
     if (offset < next_offset) {
       float f = (offset - curr_offset) / (next_offset - curr_offset);
-      vec4 curr_color = get_color(i);
-      vec4 next_color = get_color(i + 1);
+      vec4 curr_color = gsk_scaled_premultiply (get_color(i), u_alpha);
+      vec4 next_color = gsk_scaled_premultiply (get_color(i + 1), u_alpha);
       vec4 color = mix(curr_color, next_color, f);
-      gskSetScaledOutputColor(gsk_premultiply(color), u_alpha);
+      gskSetOutputColor(color);
       return;
     }
   }

testsuite/gsk/compare/conic-gradient-premultiplied-nocairo.node

@@ -0,0 +1,15 @@
+color-matrix {
+  matrix: matrix3d(2, 0, 0, 0, 0, 2, 0, 0, 0, 0, 2, 0, 0, 0, 0, 100);
+  child: container {
+    conic-gradient {
+      bounds: 0 0 50 50;
+      center: 25 25;
+      stops: 0 rgb(255,0,0), 0.6 rgba(0,255,255,0);
+    }
+    conic-gradient {
+      bounds: 0 0 50 50;
+      center: 25 25;
+      stops: 0.4 rgba(0,255,255,0), 1 rgb(255,0,0);
+    }
+  }
+}

testsuite/gsk/compare/linear-gradient-premultiplied-nocairo.node

@@ -0,0 +1,9 @@
+color-matrix {
+  matrix: matrix3d(2, 0, 0, 0, 0, 2, 0, 0, 0, 0, 2, 0, 0, 0, 0, 100);
+  child: linear-gradient {
+    bounds: 0 0 50 50;
+    start: 0 -50;
+    end: 0 100;
+    stops: 0 rgb(255,0,0), 1 rgba(0,255,255,0);
+  }
+}

testsuite/gsk/compare/radial-gradient-premultiplied-nocairo.node

@@ -0,0 +1,10 @@
+color-matrix {
+  matrix: matrix3d(2, 0, 0, 0, 0, 2, 0, 0, 0, 0, 2, 0, 0, 0, 0, 100);
+  child: radial-gradient {
+    bounds: 0 0 50 50;
+    center: 25 25;
+    hradius: 40;
+    vradius: 40;
+    stops: 0 rgb(255,0,0), 1 rgba(0,255,255,0);
+  }
+}

testsuite/gsk/meson.build

@@ -48,6 +48,7 @@ compare_render_tests = [
   'color-matrix-merge',
   'color-matrix-parsing',
   'color-states',
+  'conic-gradient-premultiplied-nocairo',
   'conic-gradient-with-64-colorstops',
   'container-single-child-offscreen-for-opacity',
   'crossfade-clip-both-children',
@@ -100,6 +101,7 @@ compare_render_tests = [
   'issue-3615',
   'linear-gradient-3d-nocairo',
   'linear-gradient-nonorthogonal-scale-nogl',
+  'linear-gradient-premultiplied-nocairo',
   'linear-gradient-with-64-colorstops',
   'lots-of-offscreens-nogl',
   'mask',
@@ -133,6 +135,7 @@ compare_render_tests = [
   'outset-shadow-scale-offset',
   'outset_shadow_simple',
   'premul-color-nogl',
+  'radial-gradient-premultiplied-nocairo',
   'radial-gradient-with-64-colorstops',
   'repeat',
   'repeating-linear-gradient-edge-colors',