renderer: completely refactor struct img

Rather than this merely representing an "in-flight" image, with the
img->sh only living for as long as this exists between different pass
types, `img` is now conceptually persistent and either in one of two
modes: `sh`, or `tex`. This allows us to, in principle, avoid doing
redundant FBO roundtrips for cases where the previous pass thinks the
next pass needs a tex but the next pass actually needs a sh, such as is
currently the case for the AV1 grain shader.

Since `pass_hook` in particular can randomly mutate `img` to either of
the two forms, callers must now be somewhat vigilant to make sure they
always use `img_tex()` and `img_sh()` to access the "current"
shader/texture, rather than relying on local variables staying
persistent.

The use of locally initialized pl_shader is now exclusive to passes that
keep their own pl_dispatch_begin calls (for various reasons).