More planets & exporting to png

Pretty big update to the generator!

4 new types of planets:

• Gas giant with ring.
• Ice World
• Lava World
• Dry Terran

I also changed the way you can choose the planet, you can now select from a dropdown menu.

Finally, An export function is now present allowing you to download a static PNG image of the current planet. Soeven if you dont use godot or don't want to use this shader, you can still use the sprites in your project.