Manual mipmapping, texture padding and filtered midtextures

Currently, mipmapping and linear texture filtering results in ugly black outlines on transparent edges. In this merge request, I fixed this by making the game generate mipmaps through a C function instead of relying on whatever OpenGL does as well as adding color padding to the transparent borders of textures when necessary. Here is what it looks like with trilinear filtering:

Before

After

The only drawback to this method I used is that it makes texture loading slower when using a texture filtering option, though I have disabled the code entirely with nearest filtering which the majority of players will probably continue sticking with anyways. I am not entirely sure about what I can do to make it faster but if anyone can give me any improvements then that would be great. After all, I am just experimenting here. Additionally, mid-textures and transparent flats are affected by texture filtering now. This might need some testing to cover all edge cases because there may be some bits of geometry that don't look quite right.