x11_common: mp_msg conversion

Doesn't touch some parts, like the X11 error handler (which doesn't
allow setting a context pointer).
gl_common: complete mp_msg conversion

Hopefully this works on Wayland and Cocoa, which I didn't test.
gl_lcms: mp_msg conversion

Have to deal with some dumb stuff in LittleCMS2's API: its error handler
is global.
aspect: mp_msg conversion
gl_osd: mp_msg conversion