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.
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