Comment 5 for bug 1236254

I think you're waiting for the wrong feature :)

The right solution for you is probably what I added in Mir 0.15.0:
    mir_connection_get_egl_pixel_format()

So all you need to do is set up your EGL context config with what you need (EGL_ALPHA_SIZE=8 or whatever) and then mir_connection_get_egl_pixel_format() will tell you what the corresponding MirPixelFormat is. More importantly, when there are apparently multiple answers, mir_connection_get_egl_pixel_format() tells you the exact MirPixelFormat that the graphics driver is really using, so as to avoid ambiguity and messed up colours (LP: #1460149)