Add a --gpu-backend option to override the gpu backend selected by Blender.

Values for this option that will be available in releases for now are:

• opengl: Force blender to select OpenGL backend.

During development and depending on compile options additional values can exist:

• metal: Force Blender to select Metal backend.
• vulkan: Force Blender to select vulkan backend.

--help text should be dynamic based on the compiled options.

When this option isn't provided the internal logic for GPU backend selection will be used.
Note that this is at the time of writing the same as always selecting the opengl backend.