The matrices that projects background images in the 3d view
were incorrect. The root cause was that the coordinate systems were not
respected, that was most noticeable when rotating a stretched image.
We re-validated conversions of coordinate spaces (UV -> Image -> Camera -> Window)
and made sure that the rotation is done in image space.
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D5431
66356dae94