This replaces the old backend that exclusively used EGL windowing with
one that can also use ANGLE's ability to render to directly to a
texture. The advantage of this is that it allows mpv to create the swap
chain itself and this allows mpv to use a flip-mode swap chain on a HWND
(which avoids problems with DirectComposition) and to use a longer swap
chain that has six backbuffers by default (which reportedly fixes
problems with rendering 24fps video on 24Hz monitors.)
Also, "screenshot window" should now work on DXGI 1.2 and up (Windows 8
and up.)
ANGLE is _really_ annoying to build. (Requires special toolchain and a
recent MSVC version.) This results in various issues with people
having trouble to build mpv against ANGLE (apparently linking it
against a prebuilt binary doesn't count, or using binaries from
potentially untrusted sources is not wanted).
Dynamically loading ANGLE is going to be a huge convenience. This commit
implements this, with special focus on keeping it source compatible to
a normal build with ANGLE linked at build-time.
James Ross-Gowan
wm4