Currently the compositor reads cryptomatte meta-data from the render
result. When loading a multilayer open exr file the meta-data was ignored.
This patch will also load the cryptomatte meta-data from multilayer open
exr files.

This enabled workflows where the rendering and compositing are done in
separate scenes or for future changes where the cryptomatte node will
use the meta-data for name matching and color picking.

Example where an openexr multilayer file is read and a cryptomatte is stored in a new file.
The meta-data is re-encoded in the new file.