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.
KOTOR: Add GUI buttons loading support.

The loading code streched out the checkbox and made it too large, so I added a
scaling factor that made the image match up the vanilla version's and
made the image extent equal to the height reported in the gui file in
order to make the checkbox round.
KOTOR: Fix coloring of button widgets.
KOTOR: Add button mouse over highlighting.
KOTOR: More button color fixes.
NWN: Properly cast variables in WidgetGridBox