morph_checkers: broken in cxx

[Starlon]

This morph plugin seems not to work with libvisual/cxx. Did something change regarding blitting with VisRectangle constraint functions?

[kaixiong]

I've just checked the code for creating sub-videos and it looks quite broken. Fixing it should fix morph_checkers.

[rigid]

changed milestone to 0.5.1 since this isn't necessarily a showstopper for the next release

[kaixiong]

Depends on #64