multimedia/libva{,-intel-driver}: clean up
- Add BUILD_DEPENDS to intel driver for wl_drm_interface fix
- Rewrite gettid() to make it easier to maintain
- Update description from upstream
- Drop unused v4l_compat dependency
- Drop unused USES=alias as <alloca.h> is gone
- Drop libGL dependency as intel driver uses libEGL via dlopen on Wayland
- Drop --program-prefix after vainfo moved to libva-utils
- Drop fmake workaround as USES=gmake is used
- Drop -ldl workaround for FreeBSD < 11.2
- Drop RTLD_NODELETE workaround for FreeBSD < 8.0
- Drop GLX_GLXEXT_VERSION workaround for Mesa < 7.8.1
- Drop Created by as the port diverged a lot (even more with Meson)
- Hint intel driver isn't actively developed (but still maintained)
- Clarify libdrm dependency