main: Major refactor of get_texture_for_framebuffer.

main: Major refactor of get_texture_for_framebuffer.

This splits off the (still) rather large chunk that is
get_texture_for_framebuffer into lots of smaller functions specialized to
service the wide variety of unique needs of *FramebufferTexture* entry points.
The result is much cleaner because, rather than having a pile of branches and
confusing conditions (like the boolean layered), the uniqueness is baked into
the entry points. The entry points know whether or not they are layered or use
a textarget.

[Fredrik: - Mention the value of <textarget> in the error message.
          - Rename check_zoffset to check_layer, and zoffset to layer.
            The zoffset parameter was renamed to layer in
            ARB_framebuffer_object.
          - Make layered a GLboolean since the value is visible to the API.
          - Remove EXT suffixes in refactored code.
          - Whitespace fixes.]

Reviewed-by: Fredrik Höglund <fredrik@kde.org>
Signed-off-by: Fredrik Höglund <fredrik@kde.org>