wasapi: Increase thread priority to reduce glitches

This is particularly important when running in exclusive mode because
any delays will immediately cause glitching.

The MinGW version in Cerbero is too old, so we can only enable this when
building with MSVC or when people build GStreamer for MSYS2 or other
MinGW-based distributions.

To force-enable this code when building with MinGW, build with
CFLAGS="-DGST_FORCE_WIN_AVRT -lavrt".

https://bugzilla.gnome.org/show_bug.cgi?id=793289
4 files changed