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
Every time?

Curiously, when I break, step, and finish in exactly the same fashion, I get 1.

```
Breakpoint 3, GScrollBarFit (gsb=0x9bca80) at gscrollbar.c:513
513	    gsb->thumbborder = GBoxBorderWidth(gsb->g.base,gsb->thumbbox);
(gdb) step
GBoxBorderWidth (gw=0x9ac9f0, box=0x7ffff6b3a420) at drawboxborder.c:1132
1132	    int scale = GDrawPointsToPixels(gw,1);
(gdb) finish
Run till exit from #0  GBoxBorderWidth (gw=0x9ac9f0, box=0x7ffff6b3a420)
    at drawboxborder.c:1132
0x00007ffff68bc0b9 in GScrollBarFit (gsb=0x9bca80) at gscrollbar.c:513
513	    gsb->thumbborder = GBoxBorderWidth(gsb->g.base,gsb->thumbbox);
Value returned is $2 = 1

```