Implemented Nearest Neighbor texture scaling.

SHA-1: 55078a729420fd531ec8feab98e79e21b5450817

* Implemented Nearest Neighbor texture scaling.

This is intended for use in games that use a lot of 2D elements that
would look very blurry when scaled with, for example, bilinear scaling,
but still use a couple 3D elements that you don't want to have
rendered/rasterized in low resolution.

Compare:
1x render size, then resized to 3x in post:
http://abload.de/img/d2-1x-nativeflqs4.png
1x render size, same as above but with SSAA active:
http://abload.de/img/d2-1x-2xssaac1pak.png
3x render size, no texture scaling (linear interpolation):
http://abload.de/img/d2-3x-linearn3rs8.png
3x render size, 3x texture size with nearest neighbor:
http://abload.de/img/d2-3x-nearestbcpbi.png

Also helps with fonts that are saved as a single big image with all
glyphs adjacent to each other.

No UI element for actually selecting this option yet.
Added nearest neighbor texture scaling to menus. (PC-style menus)
Added NN tex scaling to phone-style menus.