When we're creating folds that contain other folds, we'll need the
region based on the new fold's row mapping to overwrite the regions
from the old folds. This commit ensures that the new region cleanly
slots in, replacing any regions it completely contains and splitting
regions that it only partially overlaps.
If there are multiple mappings following the start row of the delta
that span fewer screen rows than the delta, we collapse mappings to 0
screen rows until we have removed the number of rows specified by the
delta.
Expanding positive deltas expands the first mapping following the
start row as it did previously.
Inserting a mapping should never change the position of any existing
mappings on screen or in the buffer. It's simply a statement about a
range of rows in the buffer mapping to a range of existing rows on
screen, but shouldn't add or remove any rows. Adding and removing rows
on screen or in the buffer is the job of the applyBufferDelta and
applyScreenDelta methods.
