Document how to tie together the loaded RGBA data and ICC profiles, SRGB, gamma, etc.

The color space of GdkPixbuf's loaded RGBA is not well specified. Callers are expected to figure things out from whatever pixbuf options are present; for example, if a PNG has a gAMA chunk (see !11 about actually reading that chunk), or if it provides a full iCCP chunk with an embedded color profile.

GdkPixbuf should have documentation on how clients can get all the information they need to process the RGBA data plus color space information. Maybe we need helper functions to standardize getting an ICC profile regardless of what pixbuf options a loader set.