Normally, MPV_RENDER_PARAM* arguments are copied, unless documented
otherwise. Of course we can't copy X11 Display or Wayland wl_display
types, but for arguments that are "summarized" in a struct (like
MPV_RENDER_PARAM_OPENGL_FBO), a copy is expected.
Also add some unused infrastructure to make this explicit, and to make
it easier to add parameter types that require a copy.
Untested.
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