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
Something is weird, querying bp at the line of return I get three but then gsb->thumbborder prints 69&hellip;

~~~~
(gdb) step
1133	    int bp = GDrawPointsToPixels(gw,box->border_width)+
(gdb) step
1138	return( bp );
(gdb) p bp
$32 = 3
(gdb) step
1139	}
(gdb) step
GScrollBarFit (gsb=0xabbe00) at gscrollbar.c:513
513	    gsb->thumbborder = GBoxBorderWidth(gsb->g.base,gsb->thumbbox);
(gdb) p gsb->thumbborder
$33 = 69 'E'
~~~~