resize method
Resize buffer to cover newArea. Truncates or extends as needed.
Implementation
void resize(Rect newArea) {
final length = newArea.width * newArea.height;
if (content.length > length) {
content.length = length;
} else if (content.length < length) {
content.addAll(List.generate(length - content.length, (_) => Cell()));
}
area = newArea;
}