When changing render engine, we discard the persistent data
that could be saved for all the current render instance that
exists. This is to save memory for the new renderer.
When doing so while rendering for F12, `engine_depsgraph_free`
is called after waiting for the render to finish. But this
can be called before the renderer destruction and on the main
thread.
Doing so on the main thread means that the `gpu_context` used
by the renderer cannot be bound for the sake of just receiving
the orphan buffers that the depsgraph holds. This is because
only the worker thread can make the gpu context active.
Binding the draw gpu context in this situation avoid all
possible conflict.
This is basically doing exactly what the
`DRW_render_context_enable/disable` function is doing internally
if the render engine gpu context is null.
Pull Request: https://projects.blender.org/blender/blender/pulls/129982
Clément Foucault
6312e94cec