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

@@ -5,5 +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);
 }