xvimagesink: Handle cropping when copying frames.

Cropping was not handled properly when the frames have to be copied to xvimage's buffer pool, first because the crop meta were dropped, and second because the allocated frame size in xvimage's buffer pool were smaller than the incoming frame.

This patch fixes the issue by not creating the xvimage's buffer pool in setcaps() virtual method, but it's delayed until show_frame() virtual method, when the buffer pool has to be used for copying the frame.

Also the xvimagesink's video info is updated when propose_allocation() virtual method is called.

cc: @ndufresne