fillRect method
Fills rect's area (clipped to the buffer) with char in style.
Implementation
void fillRect(int left, int top, int w, int h, String char, Style style) {
for (var y = top; y < top + h; y++) {
for (var x = left; x < left + w; x++) {
setCell(x, y, char, style);
}
}
}