[AD] al_create_custom_bitmap

[Peter Wang <novalazy@xxxxxxxxxx>]

Hi,

I still don't understand how a user is to use al_create_custom_bitmap,
except with intimate knowledge of Allegro internals.
That's not acceptable for a stable API.

If it's needed, an alternative might be to add an OpenGL-specific
(or D3D?) function which passes the upload callback a GL texture that
it can fill, or something like that.

Peter