Manually generate mipmaps rather than leaving it to the GPU

[hogsy]

This is only required for a texture atlas. Each image going onto the atlas will need it's mipmaps generated individually and then added together again.

This is going to be slightly complicated due to the textures in Hogs of War that aren't sized as powers of 2 - we may need to pad the image so it fits into the nearest power of 2 but retain the original width and height for when we add it onto the atlas.

We can use stb_image_resize for resizing.

Altogether this should resolve bleeding when enabling texture filtering with mipmaps.