Cogl doesn't use `COGL_PIXEL_FORMAT_G_8` anywhere, and it has differing
implementations depending on the driver (Mesa averages it out over the
diferent components, while NVIDIA stores it in the R component).
Furthermore, by having this new pixel format, we can use this for
single-component uploads, which will be useful when dealing with
non-trivial textures (such as YUV-based textures).
In plenty of places a non-static function was defined but didn't have
the corresponding declaration. Fix this by adding them, or alternatively
making them static.
Mutter (and libmutter users) are the only users of this version of
cogl, and will more or less only use the cogl-1.0, cogl-2.0 and cogl
experimental API variants, and having the possibility of having
different API versions of the same API depending on what file includes
it is error prone and confusing. Lets just remove the possibility of
having different versions of the same API.
https://bugzilla.gnome.org/show_bug.cgi?id=768977
