This allows us to decide when the R and B color channels should be
flipped with a much better granularity.
For instance, when using GLX_EXT_texture_from_pixmap to create a GL
texture from a surface we don't need to swap the R and B channels, as
the internal representation of the texture data will already have the
appropriate colors.
We also don't need to flip color channels when blitting from a texture.
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.
