This PR implements #126353; In short: keep discard list as part of swap chain images. This allows
better determination when resources are actually not in use anymore.
## Resource pool
Resource pools keep track of the resources for a swap chain image.
In Blender this is a bit more complicated due to the way GPUContext work. A single thread can have
multiple contexts. Some of them have a swap chain (GHOST Window) other don't (draw manager). The
resource pool should be shared between the contexts running on the same thread.
When opening multiple windows there are also multiple swap chains to consider.
### Discard pile
Resource handles that are deleted and stored in the discard pile. When we are sure that these
resources are not used on the GPU anymore these are destroyed.
### Reusable resources
There are other resources as well like:
- Descriptor sets
- Descriptor pools
## Open issues
There are some limitations that require future PRs to fix including:
- Background rendering
- Handling multiple windows
- Improve CPU/GPU synchronization
- Reuse staging buffers
Pull Request: https://projects.blender.org/blender/blender/pulls/126353
ef3ceb3629