Fix sanitized memory for rna_camera

(thanks to -fsanitize=address and Lukas Toenne ;)
Stereo output core - support rect/rectf instead of ImBufs only (for openexr multilayer)
Stereo Output for Multilayer when rendering or saving from Image Editor

We now do stereo-magic to ALL layers of multilayers.

The only remaining part of stereo output is for videos when saved with
individual views (the naming scheme at the moment is handled inside the
movie code), to be tackled later (TODO).