renderer: refactor pl_render_target.dst_rect

This is now pl_rect2df instead of pl_rect2d, to make it easier to use the pl_rect2df_aspect* series of functions, especially without requiring the hacky rounding integer versions of them. Delete those and add some needed helper functions as well.

Rewrite the fix_rects code to crop src_rect for any fractional offset in the dst_rect, and also for regions of the dst_rect that lie outside the target fbo.