The block thumb frame is now left out when the bar background is filled
which reduces flicker.
Also, the background fill draw over the region occupied with the border
lines which also caused flicker.
The border lines still draw over the thumb area, and thus, may cause
noticeable flicker there when moving it.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@13345 a95241bf-73f2-0310-859d-f6bbb57e9c96