COGL_PIXEL_FORMAT_ABGR_2101010 is defined to mean the 2 A bits are
placed in a 32 bit unsigned integer on the bits with highest
significance, followed by B on the following 10 bits, and so on, until R
on the 10 least significant bits.
UNSIGNED_INT_2_10_10_10_REV_EXT is defined to represent color channels
as
```
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
-------------------------------------------------------------------------------------
| a | b | g | r |
-------------------------------------------------------------------------------------
```
As can be seen, this matches COGL_PIXEL_FORMAT_ABGR_2101010, meaning
that's the format we can directly read and write.
In Cogl, when finding the GL formats, we get the tuple with the GL
format given the format we pass, but we also get returned "required
format" (CoglPixelFormat). This required format represents the format
that is required when reading actual pixels from GLES. In GLES, the
above mentioned format is the only one supported by the
EXT_texture_type_2_10_10_10_REV extension, thus for other types, we need
to do the CPU side conversion ourselves. To achieve this, correctly
return COGL_PIXEL_FORMAT_ABGR_2101010 as the required format.
The internal format should also be GL_RGB10_A2, not GL_RGBA.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2461>
8d179c078d