vdpau.h: Clarify video mixer field amount recommendation
vdpau.h: Clarify video mixer field amount recommendation

Reviewed-by: Stephen Warren <5ef2a23ba3aff51d1cfc8c113c1ec34b608b3b13@nvidia.com>
Signed-off-by: Anssi Hannula <a67f875f3f4d9a809275c1d14e68c85c921fe2e8@iki.fi>
Signed-off-by: Aaron Plattner <7c39395e2e801146cdbd7fb4a41282e4f38db44e@nvidia.com>

Distrotech/libvdpau,Distrotech/libvdpau,mosterta/libvdpau,mosterta/libvdpau,Distrotech/libvdpau,emericg/libvdpau-vp8,emericg/libvdpau-vp8,mosterta/libvdpau,emericg/libvdpau-vp8