This PR adds a separate "Grease Pencil" render pass. Once
the "Grease Pencil" option is checked in the passes list, the
Grease Pencil engine will render to a new render pass for various
composition uses.
Notes:
- Occluded Grease Pencil geometry is not rendered.
- In most cases, using an "Alpha Over" with the rest will result
in the same render as the "Combined" output. The exception is
when there are Grease Pencil layers that use a blending mode
that changes the chromaticity of the alpha channel.
Pull Request: https://projects.blender.org/blender/blender/pulls/137638
b05b0f413c