Restore the bigendian support that was lost in b0e26873f6,
by just not using GL_BGRA with GLES on bigendian. Should be a
very rare combination, but still.
The glyph and icon libaries were also checking for GLES to
decide if data needs to be transformed from BGRA to RGBA.
Use the new has_bgra getter instead.
This will probably break on bigendian, because the
GL_BGRA + GL_UNSIGNED_BYTE combination is not equivalent
to the cairo format on bigendian, but this was already
broken for the gl format information that we get from
gdk_memory_format_gl_format.
This moves a lot of the texture atlas control out of the driver and into
the various texture libraries through their base GskGLTextureLibrary class.
Additionally, this gives more control to libraries on allocating which can
be necessary for some tooling such as future Glyphy integration.
As part of this, the 1x1 pixel initialization is moved to the Glyph library
which is the only place where it is actually needed.
The compact vfunc now is responsible for compaction and it allows for us
to iterate the atlas hashtable a single time instead of twice as we were
doing previously.
The init_atlas vfunc is used to do per-library initialization such as
adding a 1x1 pixel in the Glyph cache used for coloring lines.
The allocate vfunc purely allocates but does no upload. This can be useful
for situations where a library wants to reuse the allocator from the
base class but does not want to actually insert a key/value entry. The
glyph library uses this for it's 1x1 pixel.
In the future, we will also likely want to decouple the rectangle packing
implementation from the atlas structure, or at least move it into a union
so that we do not allocate unused memory for alternate allocators.
