gl: Use a uniform to flip R and B colors on GLES

gl: Use a uniform to flip R and B colors on GLES

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.