I'm not sure if there are already solutions for it (high chance there are, maybe in some DMX 'simulations'), however, it shouldn't be too difficult to make your own.
Use a programming language you like which can handle graphics, use the same data structure as you would with FastLed, and let the software program (you write) show the pixels on a screen. This means you don't need any Arduino code.
If you split the filling of the pixels and the showing of it on the screen, you can copy the resulting 'filling of the pixels' code on the Arduino.
Another solution, if you want to experiment with the actual WS2812, is to program all patterns you want to check, and let inputs (via UART or fancy knobs/buttons) decide what patterns (and properties) are executed.