Adjusting offset for texture coords

Due to mipmapping the texture coordinates offset for tilesets has to change according to the level of detail chosen by the gpu. Currently the offset is lineary scaled with the final tilesetscale. In teeworlds the mipmap lod is chosen by nearest mipmap. This change tries to estimate the chosen mipmap and sets the offset according to it to increase the graphic quality. 
(Shaders are needed for a more accurate solution imho)

Set the minimum texture offset to 2 texels (provides better results than 1 texel border).
This would require to update the current tilesets with tilesetborder_add.
(Further discussion: https://www.teeworlds.com/forum/viewtopic.php?id=10280)