Function R_MipMap3D

================ R_MipMap3D Returns a new copy of the texture, eigthed in size and filtered. If a texture is intended to be used in GL_CLAMP or GL_CLAMP_TO_EDGE mode with a completely transparent border, we must prevent any blurring into the outer ring of texels by filling it with the border from the previous level. This will result in a slight shrinking of the texture as it mips, but better th