graphics/mesa*: Update to 20.2
Update to 20.2
Move graphics/libxatracker to graphics/mesa-gallium-xa
Remove VAAPI and VDPAU options in mesa-libs in favor of
two new ports : mesa-gallium-va and mesa-gallium-vdpau
Bump needed llvm to llvm10
Change a lot of meson option from false to disabled as it's
deprecated.
Remove the drm and surfaceless platform from mesa* as it's always
enabled now.
Add zstd option, used for shader cache.
Reviewed by: x11 (zeising@)
Differential Revision: https://reviews.freebsd.org/D26682