add vaapi flags as default to enable HEVC and H264(AVC)

[AnterCreeper]

on X11 use these:
--use-gl=egl
--enable-features=PlatformHEVCDecoderSupport
--disable-features=UseChromeOSDirectVideoDecoder

should work with old i965 vaapi driver(not tested with intel-media-driver, may need add LIBVA_DRIVER_NAME env)
wayland may need higher backend chromium version.

For someone from future, If read this,
if chrome 122 used by electron someday, use these below:(for X11)
--use-gl=angle
--enable-features=PlatformHEVCDecoderSupport,VaapiVideoDecodeLinuxGL