Instead of having separate callbacks for each backend-handled feature
(like MPGLContext.fullscreen, MPGLContext.border, etc.), pass the
VOCTRL responsible for this directly to the backend. This allows
removing a bunch of callbacks, that currently must be set even for
optional/lesser features (like VOCTRL_BORDER).
This requires changes to all VOs using gl_common, as well as all
backends that support gl_common.
Also introduce VOCTRL_CHECK_EVENTS. vo.check_events is now optional.
VO backends can use VOCTRL_CHECK_EVENTS instead to implementing
check_events. This has the advantage that the event handling code in
VOs doesn't have to be duplicated if vo_control() is used.
This fixes 2 bugs:
* Resizing very fast breaks the aspect of the window and the width and height
don't match with the video anymore
* Pressing 'f' for fullscreen very fast can overwrite the backup variables for
the previous width and height.
Also includes a better aspect calculation with fluid resizing.
It is now possible to show images and videos with alpha information correctly.
This was disalbed before, because there was a bug that made black parts of
videos also transparent.
Do this instead of stuffing all x11/cocoa/win32/wayland specific code
into gl_common.c. The cocoa specific parts could probably go directly
into cocoa_common.m, possibly same with wayland.
Also redo how the list of backends is managed. Get rid of the GLTYPE_
constants. Instead of having a big switch() on GLTYPE_, each backend
entry has a function pointer to setup the MPGLContext callback (e.g.
mpgl_set_backend_x11()).
