We don't want to force texture data to be premultipled if the user
explicitly specifies a non premultiplied internal_format such as
COGL_PIXEL_FORMAT_RGBA_8888. So now Cogl will only automatically
premultiply data when COGL_PIXEL_FORMAT_ANY is given for the
internal_format, or a premultiplied internal format such as
COGL_PIXEL_FORMAT_RGBA_8888_PRE is requested but non-premultiplied source
data is given.
This approach is consistent with OpenVG image formats which have already
influenced Cogl's pixel format semantics.
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