shaders: initialize vertexes once

Create a buffer for the vertex data and place vertexes
there at initialization time.  Then just use the buffer
for each texture blit.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
