transition_composite.c: add animatable geometry "pan" property. This suppresses impli...

transition_composite.c: add animatable geometry "pan" property. This suppresses implicit scaling of the b-frame and makes the compositing rectangle crop. Then, it uses the x and y geometry information to pan the b-frame within the composite rectangle. For example, a negative x pans the image to the left and that portion of the b frame left of the composite rectangle is cropped. w and h of the pan geometry is not implemented at this time, but the plan is to implement scaling of the b-frame. In the end, this can provide a Ken Burns effect for still images - a much requested feature.

git-svn-id: https://mlt.svn.sourceforge.net/svnroot/mlt/trunk/mlt@1136 d19143bc-622f-0410-bfdd-b5b2a6649095