Add texture filtering option to terrain material

[jacobcoughenour]

I added the option to change the texture filtering mode the shader uses from the TerrainMaterial3D. This allows me to have the sharp pixelated look without having to implement my own shader.

Here's what the options look like:

LINEAR - uses linear_mipmap_anisotropic

NEAREST - uses nearest_mipmap_anisotropic

[TokisanGames]

Thank you.