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
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GBoxBorderWidth (gw=0x887b00, box=0x7ffff6b213a0 <scrollbar_box>)
    at drawboxborder.c:1132
1132	    int scale = GDrawPointsToPixels(gw,1);
(gdb) p scale
$1 = -9136
(gdb) step
GDrawPointsToPixels (gw=0x887b00, points=1) at gdraw.c:61
61	    if ( gw==NULL ) {
(gdb) step
66	return( PointToPixel(points,gw->display->res));
(gdb) p gw->display->res
$2 = 100
(gdb) p points
$3 = 1

~~~~