fbo-blit: Test for accurate proportions during significant scalage/shrinkage with and without clipping

The following test helped detecting incorrect rendering on blitting, when floats were replaced with integers in this patch, while the rest of CI tests passed.

Covers cases of scaling up and shrinking down (both with and without clipping source), combined with mirroring and clipping to/from very narrow region (which may significantly affect proportions of resulting image).