We spend a lot of time invalidating in the DX Renderer. This is a
creative trick to not bother invalidating any further if we can tell
that the bitmap is already completely invalidated. That is, if we've
scrolled at least an entire screen in height... then the entire bitmap
had to have been marked as invalid as the new areas were "uncovered" by
the `InvalidateScroll` command. So further setting invalid bits on top
of a fully invalid map is pointless.
Note: I didn't use `bitmap::all()` here because it is significantly
slower to check all the bits than it is to just reason out that the
bitmap was already fully marked.
## Validation Steps Performed
- Run `time cat big.txt`. Checked average time before/after, WPR traces
before/after.