[gtk+/wip/ebassi/gsk-renderer] gsk: Add more modes to the blend shader

[Emmanuele Bassi <ebassi src gnome org>]

commit fc27c9d4f85f61b7bd101eb7d3431941e7f406ed
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Sun Jul 3 23:02:56 2016 +0100

    gsk: Add more modes to the blend shader
    
    Use the compositing CSS spec at:
    
      https://www.w3.org/TR/compositing-1/#blending
    
    For the implementation.

 gsk/resources/glsl/blend.fs.glsl |   17 +++++++++++++++++
 1 files changed, 17 insertions(+), 0 deletions(-)
---
diff --git a/gsk/resources/glsl/blend.fs.glsl b/gsk/resources/glsl/blend.fs.glsl
index 5d2f873..ddc8d95 100644
--- a/gsk/resources/glsl/blend.fs.glsl
+++ b/gsk/resources/glsl/blend.fs.glsl
@@ -2,6 +2,17 @@ vec3 BlendMultiply(vec3 Csrc, vec3 Cdst) {
   return Csrc * Cdst;
 }
 
+vec3 BlendScreen(vec3 Csrc, vec3 Cdst) {
+  return Csrc + Cdst - (Csrc * Cdst);
+}
+
+vec3 BlendHardLight(vec3 Csrc, vec3 Cdst) {
+  vec3 m = BlendMultiply(Csrc, 2.0 * Cdst);
+  vec3 s = BlendScreen(Csrc, 2.0 * Cdst - 1.0);
+  vec3 e = vec3(0.5, 0.5, 0.5);
+  return mix(m, s, step(e, Cdst));
+}
+
 void main() {
   vec4 src = Texture(map, vUv);
   vec4 mask = Texture(parentMap, vUv);
@@ -13,6 +24,12 @@ void main() {
   else if (blendMode == 1) {
     res = BlendMultiply(src.xyz, mask.xyz);
   }
+  else if (blendMode == 2) {
+    res = BlendScreen(src.xyz, mask.xyz);
+  }
+  else if (blendMode == 8) {
+    res = BlendHardLight(src.xyz, mask.xyz);
+  }
   else {
     // Use red for debugging missing blend modes
     res = vec3(1.0, 0.0, 0.0);