st/vdpau: only send buffers with B8G8R8A8 format to X

st/vdpau: only send buffers with B8G8R8A8 format to X

PresentPixmap only works if the pixmap depth matches with the
window depth, otherwise it returns a BadMatch protocol error.
Even if the depths match, the result won't look correctly
if the VDPAU RGB component order doesn't match the X11 one so
we only allow the X11 format.
For other buffers we copy them to a buffer which is send to X.

v2: only send buffers with format VDP_RGBA_FORMAT_B8G8R8A8
v3: reword commit message
v4: add comment explaining the code

Signed-off-by: Nayan Deshmukh <nayan26deshmukh@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>