Whether this rounded rectangle is a simple rectangle with zero corner radii.
Source
bool get isRect { return (tlRadiusX == 0.0 || tlRadiusY == 0.0) && (trRadiusX == 0.0 || trRadiusY == 0.0) && (blRadiusX == 0.0 || blRadiusY == 0.0) && (brRadiusX == 0.0 || brRadiusY == 0.0); }