wayland/shm: use opaque regions

Make use of opaque regions on non-alpha formats. This allows the compositor to
improve the drawing of the surface, because he can discard everything behind
the window when drawing.
wayland: don't create our own log context

This was more problematic than useful ([vo/wayland/wayland])
configure: move wayland-egl check

This makes it possible to build the shm backend when no EGL platform is
available.
configure: fix VDA autodetection based on FFmpeg support

The original condition was too weak, requiring only the header. The header is
installed is FFmpeg regardless of the presence of VDA on the system, so just
perform a check on the `ff_vda_create_decoder` function.
cocoa_common: autohide dock when autohiding menubar

A cocoa bug doesn't allow to do otherwise. Will open a radar later.
Merge remote-tracking branch 'origin/master' into lua_experiment
wayland: fix memory leaks
wayland/shm: rework format handling

Use a linked list for all supported formats and make the format table const.
osc: reduce scaling of OSD symbols to fit new fontmetrics

Apparently, the OSD-symbol-font had somewhat broken metrics before, causing the symbols to be rendered too large. So far this was simply compensated by applying a \fs-5 tag to them (see https://github.com/mpv-player/mpv/blob/master/sub/osd_libass.c#L40 ). Now that the font has been run through a proper editor, it seems the metrics got fixed along the way making the symbols render way to small due to the \fs-5 tag. Changing it to \fs-1 fixed this.
Merge remote-tracking branch 'origin/master' into lua_experiment