Fix for crash when using 2D stabilization for float movie clips

Also removed unneeded image buffer scaling, it was only needed
for "early output" if there was no rotation. That is no longer
supported since it used to pixelate result a lot and interpolation
is always used now.

Saves quite a few of memory and CPU cycles.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56517 954f8c5b-7b00-dc11-b283-0030488c597c