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
Latest master gives me 69 from it:

~~~~
(gdb) step
GBoxBorderWidth (gw=0x8d1520, box=0x7ffff6b213e0 <thumb_box>)
    at drawboxborder.c:1132
1132	    int scale = GDrawPointsToPixels(gw,1);
(gdb) finish
Run till exit from #0  GBoxBorderWidth (gw=0x8d1520, 
    box=0x7ffff6b213e0 <thumb_box>) at drawboxborder.c:1132
0x00007ffff68a28f6 in GScrollBarFit (gsb=0x8f19f0) at gscrollbar.c:513
513	    gsb->thumbborder = GBoxBorderWidth(gsb->g.base,gsb->thumbbox);
Value returned is $2 = 69
(gdb) 
~~~~