[premultiplication] Be more conservative with what data gets premultiplied

[premultiplication] Be more conservative with what data gets premultiplied

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.