This should get mpv working on Windows 7 machines without hardware
accelerated graphics adapters. It already worked on Windows 8 and up
because those systems would silently fall back to WARP if there was no
graphics hardware installed.
The normal MPGL_CAP_SW flag is not set, so unlike other opengl backends,
this will choose a software adapter even if opengl:sw is not specified.
The reason for this is, unlike on Linux, where vo_xv and vo_x11 can be
used, mpv on Windows does not have any VO to fall back on when hardware
acceleration isn't available, so if software adapters are rejected, the
user won't see any video output when using the default settings. WARP
seems to perform quite well, so it should be used in this case.
43e811cb4b