When using grease pencil in a stereo rendering the grease pencil objects are
only visible in the left eye. In the viewport it renders both.
Issue is related that `DRW_render_gpencil` only renders a single view. But
`DRW_render_to_image` renders all views. This patch puts this in a loop to
render both eyes.
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D7154
8ba9efb7d8