This refactor part of `draw_manager_c.cc` to make it more understandable
and less bug prone.
- Splits the context handing to `draw_gpu_context.cc`
- Rename `draw_manager_c.cc` to `draw_context.cc`
- Merge `DRWContextState` into `DRWContext`
- Merge lots of static functions into `DRWContext` to avoid global access
- Deduplicate code between entry point functions
- Move context init logic to `DRWContext` constructor
- Move resource init logic to `DRWContext::acquire_data`
- Move extraction `TaskGraph` out of `DRWContext`
- Reduce / centralize complexity of enabling draw engines
- Reduce the amount of `drw_get` calls
- Remove unused code
Pull Request: https://projects.blender.org/blender/blender/pulls/135821
94c7c84bcd