[gtk+/wip/ebassi/gles] gl: Tweak the swizzle for GLES texture fragments

[Emmanuele Bassi <ebassi src gnome org>]

commit 19080ac2ef3a7d97eb937a58a87f12c2cce631d4
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Sat Apr 23 13:52:03 2016 +0100

    gl: Tweak the swizzle for GLES texture fragments
    
    Cairo surfaces are in BGRA format, but we upload them as RGBA buffers on
    GLES; this means that the R and B channels are flipped in the texture
    data.
    
    Instead of doing a costly channel flip before putting them on the GPU,
    we can flip the values inside the GLSL shader we use specifically for
    GLES.

 gdk/resources/glsl/gles2-texture.fs.glsl |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/gdk/resources/glsl/gles2-texture.fs.glsl b/gdk/resources/glsl/gles2-texture.fs.glsl
index 68e455f..56c6c82 100644
--- a/gdk/resources/glsl/gles2-texture.fs.glsl
+++ b/gdk/resources/glsl/gles2-texture.fs.glsl
@@ -5,5 +5,8 @@ uniform sampler2D map;
 varying highp vec2 vUv;
 
 void main() {
-  gl_FragColor = texture2D(map, vUv);
+  vec4 color = texture2D(map, vUv);
+
+  /* Flip R and B around to match the Cairo convention */
+  gl_FragColor = vec4(color.z, color.y, color.x, color.w);
 }