Refactoring

[Mufalder]

Did some refactoring and clean-ups.
Here is the list of all things that has been done:

• Upgraded project to the 2020.3.17f1 version
• Code cleaning, moved repeating code to the .cginc file
• Removed the vertex cutout by the fog distance
• Removed custom fog calculation and used Unity builtin functions
• Customizable vertex precision
• More universal shaders with shader features and enums (4 shaders instead of 13)
• Added lightmapping support
• Cull selection option
• Instancing support