linedisplaycache: improve invalidation with y_range

linedisplaycache: improve invalidation with y_range

When we invalidate a y_range using the common pattern of y==0 and
old_height==new_height, we are generally invalidating the entire buffer.

This short-circuits that case to just invalidate the buffer in a faster
and more complete form. The problem here appears to be that we can't
always calculate the ranges properly to invalidate because validation
has not run far enough.