Expose our `BKE_image_load_exists` feature through an optional parameter to `Image.load()`.
Extend `image_utils.load_image()` with two optional parameters, to return existing image datablock
if possible, and in that case, to force reloading said image.
Needed by incomming 'import images as planes' addon enhancement.
- add rna property 'as_bytes' method so you can get a string property as python bytes (bypass encoding).
- make bpy.path.abspath/relpath compatible with bytes.
- add 'relpath' option to bpy_extras.image_utils.load_image(), so you can load an image relative to a path.
in existing addons this only effects OBJ import.
interestingly even though this is a bug (in that its returning a placeholder when it should load the image), since the placeholder points to the correct path, the image will load correctly when refreshed, so the bug's not too bad.
