Fix gdraw scrollbars

- Subtract the offset height of the thumb to the total height before calculating the thumb pos by ratio
- Highly decrease the minimum size for the scrollbar thumb &ndash; worked-around with a constant setting since the computation [happening here](https://github.com/fontforge/fontforge/blob/a7886343df283b2dbf33562e43835c9d0cf47a2f/gdraw/gscrollbar.c#L513) is quite obfuscated and warrants deeper research.

cc #1552 (for the first part), #1583, #1233.

r? @frank-trampe
Tried two times in a row, then another time after relaunching and got 69 each time. I think I got 67 on my laptop.