By receiving the graphics API-specific contexts from the generic
pl_gpu instance, it is not necessary to store them separately.
As the current priv struct stores nothing else, this allows its
removal. This removal cleans up related compiler warnings regarding
unused variables in case of opengl and d3d11 being disabled in
mpv/libplacebo.
Functions to receive the underlying vulkan,gl,d3d11 structure were
added in libplacebo version 4.182. Our current requirement for
gpu-next is 4.190, and thus we can freely utilize these helper
functions.
This is done to avoid cluttering vo_gpu_next.c with more ifdeffery and context-specific code
when additional backends are added in the near future.
Eventually gpu_ctx is intended to take the place of ra_ctx to further separate gpu and gpu_next.