[enhancement] Add API for querying attributes of a MirPixelFormat

This was the original intention behind the format class, so I'm all for this enhancement

I now think a simple query function might be better than modifying the enum because 32 bits (typical enum size) isn't really enough space to store all the information we want. A simple query function could just return a const pointer to a struct that describes the pixel format in detail.

Low priority because any self-respecting software client that wants decent performance will always still need separate optimized code paths.

I want to raise priority on this as Qt allows clients to select different pixel formats at runtime, and needs to apply heuristics to select the closest matching mir pixel format. Right now to achieve this, we have to hardcode the mir pixelformats which have an alpha channel.

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)

Syncing task from Mir.