The vp_w/vp_h variables and parameters were not really used anymore
(they were redundant with ra_tex w/h) - but vp_h was still used to
identify whether rendering should be done mirrored.
Simplify this by adding a fbodst struct (some bad naming), which
contains the render target texture, and some parameters how it should be
rendered to (for now only flipping). It would not be appropriate to make
this a member of ra_tex, so it's a separate struct.
Introduces a weird regression for the first frame rendered after
interpolation is toggled at runtime, but seems to work otherwise. This
is possibly due to the change that blit() now mirrors, instead of just
copying. (This is also why ra_fns.blit is changed.)
Fixes#4719.
346ac1e09f