setViewport method
Implementation
void setViewport(double viewportWidth, double viewportHeight) {
assert(viewportWidth > 0, "viewportWidth cannot be 0 or lower");
assert(viewportHeight > 0, "viewportHeight cannot be 0 or lower");
final cx = centerX, cy = centerY;
_viewportWidth = viewportWidth;
_viewportHeight = viewportHeight;
centerX = cx;
centerY = cy;
}