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.
