As we do lots of writes to *Iptr, the compiler isn't allowed to cache any shared variable (neither index nor Iptr itself).
This commit inlines Iptr + index into the index generator functions, so the compiler know that they are const.
We are used to render them out of order as long as everything else matches, but rendering order does matter, so we have to flush on primitive switch. This commit implements this flush.
Also as we flush on primitive switch, we don't have to create three different index buffers. All indices are now stored in one buffer.
This will slow down games which switch often primitive types (eg ztp), but it should be more accurate.
