Blender Git Loki

Avoid imbuf loading when getting movie clip size

This prevents high memory usage by non-proxied frames when doing mask parenting.

Description from code:

Originally was needed to support image sequences with different image dimensions,
which might be useful for such things as reconstruction of unordered image sequence,
or painting/rotoscoping of non-equal-sized images, but this ended up in unneeded
cache lookups and even unwanted non-proxied files loading when doing mask parenting,
so let's disable this for now and assume image sequence consists of images with equal sizes