By providing an (internal) table to map `CoglPixelFormat`s to their
respective properties we will be able to get rid of the unusual enum
values in the future. This is something we will need once we want to
have support for more pixel formats (such as YUV-based formats).
As an extra feature, we provide a `to_string()` method, which is quite
useful for debugging purposes (rather than deciphering enum values).
https://gitlab.gnome.org/GNOME/mutter/merge_requests/524
We're going to add some features and extra code to CoglPixelFormat, so
it's much nicer to have it in once place. Notice also that it doesn't
make sense that e.g. `_cogl_pixel_format_get_bytes_per_pixel()` were in
a private header, since they were being exported anyway.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/524
