renderer: fix possible compute to non-storable fbo

The scaler code only checks whether or not the FBOFMT is storable,
but in the case of e.g. draw_overlays, we have to check the actual FBO
(provided by the user) instead.

Solve this by just checking both. (Note: In theory, if the FBO is
storable but FBOFMT is not, this would be a false negative. But
whatever.)