measureFont method
void
measureFont()
Measure font metrics so cells are sized correctly.
Implementation
void measureFont() {
context.font = '${fontSize}px $fontFamily';
final m = context.measureText('M');
_cellWidth = m.width;
if (_cellWidth <= 0) _cellWidth = 8;
_cellWidth = _cellWidth.ceilToDouble();
_cellHeight = (fontSize * 1.2).ceilToDouble();
_baseline = (fontSize * 0.85).ceilToDouble();
}