renderer: merge pl_image and pl_render_target into pl_frame

This is motivated by these two structs, essentially, having
conglomerated into unity. Drop the deprecated fields from each and just
go ahead with the big search & replace.

This also allows us to unify a couple of helper functions, quite
pleasingly. The only really annoying thing is that `target.av1_grain`
makes zero sense, but whatever.

We also provide a new helper function to bridge the "functionality gap"
in the `pl_render_target_partial` / `pl_tex_clear` checks.

An attempt was made to preserve backwards compatibility, for what it's
worth.