GPU backend selection

Keep in sync with https://hackmd.io/@55n-91bZTVmcvTOc9vZV5Q/r1BtJMIdo and eventually upload the plantuml diagrams here as images.

Requirements

• From the command line it should be possible to override the GPU backend selection.
• From the user preferences it should be possible to override the GPU backend selection.
• Blender should try multiple backends to find a matching backend.
• When an unsupported override is set blender should fallback to OpenGL.
• The command line argument has more priority than the user preference setting.

Current API

• GPU_context_create creates a new context based on GPU_backend_type_selection_get.
  • GPU_context_create requires the correct ghost_context for the current GPU_backend_type_selection_get. Expect crashes when this isn't enforced.
• GPU_backend_supported checks if the backend returned by GPU_backend_type_selection_get is supported by the platform. It doesn't require the context to be created.
• GPU_backend_type_selection_set overrides the default backend with the given type. It is called during command line argument parsing for the --gpu-backend argument. It is also called by ShaderBuilder and test cases to check multiple backends during the same run.
• GPU_backend_get_type returns the backend type that was initialized when the context is created.

Proposal

User Preferences

In the user preferences system tab a new panel will be added "GPU Backend" with the next options:

• Default
• OpenGL
• Metal

For now the panel only appears on Apple

Changing this value requires a restart. This should be clearly communicated to the user.

Technical the user could set an override to something that isn't supported by his system. For example when copying/syncing user preferences to other machines. This could be detected and automatically switch back to the default behavior.

Impact

User preference

• Add gpu_backend to UserDef structure in DNA_userdef_types.h
• In rna_userdef.c define gpu_backend property.
• Add USERPREF_PT_system_gpu_backend to space_userpref.py.

GPU module

• Implement the new API functions and tweak the existing ones.
• Cache the result of GPU_backend_supported until GPU_backend_type_selection_set is called.

Command line arguments

• In creator_args.c#arg_handle_gpu_backend_set use GPU_backend_type_selection_set_override.

Window manager

• In wm_init_userdef check if backend types are not yet overridden. If so override the backend types with the setting stored in the user preference.
• In wm_init_userdeflet the GPU_module detect which GPU backend should be used.