global: eliminate `priv` indirections

This has some advantages:

• one fewer pointer to dereference per object
• leaks fewer internal implementation details

And some drawbacks:

• can no longer peek inside pl_pass_params.target_dummy, but this was not really useful anyways
• dummy.c got slightly uglier
• pl_fmt.priv can no longer directly point at the underlying fmt
• can no longer use priv structs as talloc contexts (but this shouldn't be done anyway, with vk being the exception)

In addition to eliminating priv, I also decided to eliminate the pl_gpu_fns indirection. pl_gpus are now directly "cast down" to pl_gpu_fns by the gpu.c helpers.