If the GL implementation/hw supports the GL_*_texture_swizzle extension,
pretend that BGRA textures shall contain RGBA data, and let the flipping
happen when the texture will be used in the rendering pipeline.

This avoids rather expensive format conversions when forcing BGRA buffers
into RGBA textures, which happens rather often with WL_SHM_FORMAT_ARGB8888
buffers (like gtk+ uses) in little-endian machines.

In intel/mesa/wayland, the performance improvement is rather noticeable,
CPU% as seen by top decreases from 45-50% to 25-30% when running
gtk+/tests/scrolling-performance with a cairo renderer.

https://bugzilla.gnome.org/show_bug.cgi?id=779234