Fix drawing of tree lines

[hermannoffen]

• changed method TBaseVirtualTree.DrawLineImage to not wrongly apply (scaled) FImagesMargin anymore
• instead (if not Reverse) simply use FIndent decreased by one pixel to draw the line just next to a node

[hermannoffen]

The problem is currently not observable very prominently, but it is when private field FImagesMargin's value is increased or decreased in a significant way.

Check the reproduction demo of #1191 (after applying its corresping pull request #1192). A more detailed inspection is possible when setting LineStyle to lsSolid.

1. Without this fix and a default FImagesMargin of 2, horizontal tree lines reach 3 pixels into the node's rect:
   
2. Without this fix and a much bigger FImagesMargin of 20, horizontal tree lines reach way too much into the nodes rect:
   
3. With this fix applied, horizontal tree lines are ending just at the node's content rect, as expected:
   

FImagesMargin shouldn't affect the line drawing.