GRAPHICS: Treat FreeType's FT_Bitmap fields as unsigned

This fixes compilation for FreeType 2.5.4:
> Some fields in the `FT_Bitmap' structure have been changed from
> signed to unsigned type, which better reflects the actual usage.
> It is also an additional means to protect against malformed input.
>
> This change doesn't break the ABI; however, it might cause compiler
> warnings.

In our case, since we're using a MIN() template, it even caused
compilation to fail.

Treating them as unsigned is apparently the preferred way, so we're
always doing that now, even for FreeType < 2.5.4.