st/vdpau: fix vlVdpOutputSurfaceRender(Output|Bitmap)Surface

Correctly handle that the source_surface is only optional.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80561

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit b73c20759fe1b4a96296bed58637f5ded7c9155c)
vl/compositor: set the scissor before clearing the render target

Otherwise we clear areas that shouldn't be cleared.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 03a99ba9e41ea86355a4febbe0b1a114d5455b9f)