SDL demo released

I have switched the project to SDL3 and released a demo. It has a smoother frame rate and I also slightly improve the animation effect by a little. The trade-off is slightly higher GPU usage, which I am trying my best to optimize.

In the next step, I will try to add some texts for slay and hit counts. I expect this again to increase the GPU usage slightly.

I'll try to update this project daily recently until it is in good shape.

New staffs:

- Random slime colors.

- Better hit effect and attack animation.

- Slime wandering around now.