Switch font texture U/V coordinates to unsigned int

This fixes build with freetype 2.5.4, in which FT_Bitmap::rows,
FT_Bitmap::width types were changed from int to unsigned int. Because
of that, max(int, unsigned int) failed to compile. And since U/V
coordinates are essentially unsigned, this also improves code
consistency.
Merge pull request #3273 from AMDmi3/freetype-2.5.4

Switch font texture U/V coordinates to unsigned int