VAAPI: Add check for maximum surface count in EnsureSurfaces

EnsureSurfaces does not check if the requested number of surfaces exceeds the hardcoded maximum count.
If for example the renderer stalls and keeps too many surfaces, it'll keep allocating more surfaces, and at one point start writing beyond the maximum 32 array-elements, which causes weird crashes.

This patch simply adds a check to EnsureSurfaces, so it just fails if too many surfaces are requested.

@fritsch
@elupus