setFPS method
Changes the target FPS backend renders at.
No-op if newFPS equals the current fps. Otherwise updates backend's
target FPS, then notifies the app about the FPS change and emits
EventFPSChanged with the old and new values.
Implementation
void setFPS(int newFPS) {
if (newFPS == _fps) return;
app.backend.setTargetFPS(newFPS);
int oldFPS = _fps;
_fps = newFPS;
app._doFPSChange(oldFPS, newFPS);
app.emit(EventFPSChanged(app, oldFPS, newFPS));
}