XCB/XVideo: request correct picture format from the adaptor

XCB/XVideo: request correct picture format from the adaptor

This is bug compatibility for XVideo drivers that scale the whole
picture rather than the source rectangle (the XVideo 2 specification
does not really state which behaviour is correct).

This works because the video output core only applies crop parameters
*after* the vout display plugin is activated. This is brittle though:
the A/R and crop parameters should probably be passed as configuration
like zoom.