This is pretty trivial, but also quite annoying due to details like
mismatching eglGetProcAddress() function signature (most callers just
cast the function pointer), and ARM/Linux hacks. So move them all to one
place.
The way it should (probably) work is that selecting a RGBA framebuffer
format will simply make the compositor use the alpha. It works this way
on Wayland. On X11, this is... not done. Instead, both GLX and EGL
report two FB configs, which are exactly the same, except for the
platform-specific visual. Only the latter (non-default) points to a
visual that actually has alpha. So you can't make the pure GLX and EGL
APIs select alpha mode, and you have to override manually.
Or in other words, alpha was hacked violently into X11, in a way that
doesn't really make sense for the sake of compatibility, and forces API
users to wade through metaphorical cow shit to deal with it.
To be fair, some other platforms actually also require you to enable
alpha explicitly (rather than looking at the framebuffer type), but they
skip the metaphorical cow shit step.
Add a function to egl_helpers.c for creating an EGL context and make
context_x11egl.c use it. This is meant to be generic, and should work
with other windowing APIs as well. The other EGL-using code in mpv can
be switched to it.
Until now, we've used system-specific API (GLX, EGL, etc.) to retrieve
the depth of the default framebuffer. (We equal this to display depth
and use the determined depth for dithering.)
We can actually retrieve this value through standard GL API, and it
works everywhere (except GLES 2 of course). This simplifies everything a
great deal.
egl_helpers.c is empty now. But I expect that some EGL boilerplate will
be moved to it, so don't remove it yet.
Do this to make the license situation less confusing.
This change should be of no consequence, since LGPL is compatible with
GPL anyway, and making it LGPL-only does not restrict the use with GPL
code.
Additionally, the wording implies that this is allowed, and that we can
just remove the GPL part.
