In recent refactoring {a9f2ebb21508} an issue was introduced that the
opengl rasterizer would be disabled when only writing to a stencil
buffer.
This fix adds stencil writing to the write mask and set it. This makes
the write map not evaluate to GPU_WRITE_NONE and the rasterizer will be
enabled in `GLStateManager::set_write_mask`.
Reviewed By: Clément Foucault
Differential Revision: https://developer.blender.org/D8743