main: Split framebuffer_texture.

main: Split framebuffer_texture.

Split apart utility function framebuffer_texture to better prepare for
implementing NamedFramebufferTexture and NamedFramebufferTextureLayer.  This
should also pave the way for some future cleanup work.

[Fredrik: - Mention which limit was exceeded when <layer> is out of range.
          - Update a comment to reflect that <fb> might not be the bound
            framebuffer.
          - Make it clear that the error message in glFramebufferTexture*D
            refers to the <textarget> parameter.
          - Remove EXT suffixes.]

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