Blender Git Loki

DrawManager/GreasePencil: Select Correct ViewLayer

The DrawManager assumed that the first render layer was the render layer
where GP needed to render on. In viewport this is always the case as we
only have a single view layer. When rendering this can be multiple when
multiple layers are rendered and composited in the compositor.

In stead of the assumption that the first render layer is the render
layer we need to draw on, we search for the render layer with the same
name as the viewlayer.

Reviewed By: fclem, brecht, antoniov

Maniphest Tasks: T63099

Differential Revision: https://developer.blender.org/D4818