This commit fixes the following assert:
mtl_command_buffer.mm:165, submit(), at 'MTLBackend::get()->is_inside_render_boundary()'
It happens when toggling rendered state of viewport on macOS, and is
caused by incorrect order of setting active GPU context to null and
calling GPU_render_end.
This change makes the flow of GPU_render_{beign, end} and GPU context
activation closer to what it is in the draw manager's functions
DRW_render_context_{enable, disable}.
Pull Request: https://projects.blender.org/blender/blender/pulls/119868
7e2d54e786