This fixes a bug that would clear the cells with the current template
cell with just the `flags` reset, to make sure the colors are correct.
However, the cell foreground was not reset, leading to cells counting as
occupied when resizing.
With this change both cell flags and foreground color are ignored when
clearing both the whole screen and inside the line, allowing us to
accurately keep track of cell occupation.
Fixes#2866.
Kirill Chibisov
Christian Duerr
Theodore Dubois