Fixes for keyframe selection
Fixes for keyframe selection

Using tracks with constant zero weight used to crash
keyframe selection since it was trying to use missing
parameter blocks for Jacobian evaluation,

Also fixed possible issues with wrong camera block being
marked as variable. This could technically happen when
having zero weighted tracks. Made it so all camera blocks
are marked as variable for now.

petertsoi/libmv,petertsoi/libmv,rgkoo/libmv-blender,rgkoo/libmv-blender,rgkoo/libmv-blender,SoylentGraham/libmv,SoylentGraham/libmv,petertsoi/libmv,SoylentGraham/libmv,rgkoo/libmv-blender,SoylentGraham/libmv,rgkoo/libmv-blender,petertsoi/libmv