Currently when rendering the view layer of a grease pencil layer that has
a mask layer attached, the mask layer would show in the rendered image.
This is inconsistent with the default behaviour with no mask on the
grease pencil layer, because it would only render what's on that
particular layer and not anything from any other layer.

This patch makes the masks invisible in the render.

Note: This might seem like not the best solution, but because masks are
just regular grease pencil layers, it's tricky to pass this edge-case to the
drawing code. The way it is handled right now is the best I could come
up with, without making changes that could affect something else.