We did have 4 ordering variations of ARGB straight,
but only 3 premultiplied. Add the missing one.
Update all the places where we switch over memory formats.
We need them for mask-only textures.
For tiffs, we convert the formats to RGBA (the idea that tiff can save
everything needs to be buried I guess) as tiffs can't do alpha-only.
Public headers should mainly include gdktypes.h, which already include
the symbol visibility and versioning macros; we can also modify
gdktypes.h to include the enumerations.
