gsk: Use has_bgra in more places
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 should already be broken for the gl format information that we get from gdk_memory_format_gl_format.