This means we don't have to explicitly set the foreground/background
to the grid's default colors whenever we reset/clear a cell, and we
can instead simply memset() the entire cell to 0.
This also means the renderer has to get the default color when
rendering a cell without a foreground/background color set.
Vim, for example, changes the scroll region every time you scroll a
single line. Thus, resetting the damage queue is slow.
This reworks the damage handling of scroll updates:
* Split damage queue into two: one for scroll operations and one for
update/erase operations.
* Don't separate update/erase operations inside/outside the scroll
region
* Store the current scroll region in the scroll damage operation. This
allows us to stack multiple scroll operations with different scroll
regions.
* When updating update/erase operations after a scroll operation,
split the update/erase operations if necessary (the current scroll
operation may have a scroll region different from before, thus
forcing us to split existing update/erase operations.
* The renderer no longer erases after a scroll. The scroll operation
also adds an erase operation. This also means that erase operation
are subject to adjustments by later scroll operations.
This is largely untested, but existing scrolling code has been
converted to using a terminal-global scrolling region that is defined
as start-end of the scrollable region.
This is compared to the old code where the scrolling region where
defined in terms of marginals, counted in lines from top and from
bottom.
Instead of having each cell in the grid track it's own dirtiness, grid
operations now append "damage" to a list.
This list is consumed every time we render the grid.
This allows us to special case some operations, like erase (and in the
future, scroll).
