This PR adds the drawing placement modes from GPv2.
The drawing placement defines the depth (origin, view, surface, etc.) and a plane (view, cursor, xz, etc.).
This introduces a new helper class `DrawingPlacement` that does all of the internals to find the correct projection and just exposes a simple function to project from screen space to the drawing plane/surface.
Note: Drawing on other strokes currently doesn't work, because GPv3 can't be rendered to image yet. We use the depth buffer of the grease pencil render result to find the right depth.
Pull Request: https://projects.blender.org/blender/blender/pulls/115602
5fea1eda36