Method TBaseVirtualTree.GetOffsets unnecessarily applies FImagesMargin in most cases

[hermannoffen]

Problem

Method TBaseVirtualTree.GetOffsets (introduced in #369) incorrectly applies FImagesMargin for TVTElement.ofsCheckBox when no check image is needed. Thus, its out-param pOffsets then contains values two pixels too high for TVTElement.ofsCheckBox and all following elements.

Observations

This can, for example, cause a wrong result of TBaseVirtualTree.GetMaxColumnWidth, making autosized columns wider than necessary.
It might also be causing #811, I guess.

Reproduction

It's nicely reproducible by extending Minimal demo project:

1. Get write access to private field FImagesMargin using the with Self do class helper hack:

type
  TVirtualTreeHelper = class helper for TBaseVirtualTree
    procedure SetImagesMargin(const AValue: Integer);
  end;

procedure TVirtualTreeHelper.SetImagesMargin(const AValue: Integer);
begin
  with Self do
    FImagesMargin := AValue;
end;

2. Add a TSpinEdit control to TMainForm.dfm and set its Value to 2 as it is the hard coded value of FImagesMargin:

  object SpinEdit1: TSpinEdit
    Value = 2
  end

3. Implement the spin edit's OnChangeEvent to apply its value to FImagesMargin:

procedure TMainForm.SpinEdit1Change(Sender: TObject);
begin
  VST.SetImagesMargin(SpinEdit1.Value);
  VST.Invalidate;
end;

4. Compile and start to initially see the unchanged behavior:
   
5. Use the spin edit to change FImagesMargin and unexpectibly see the nodes' labels move to the right when increasing it or to the left when decreasing it:
   
6. As there are no images visible at all, FImagesMargin shouldn't change the display at all.

[joachimmarder]

Thank you for sending the pull request. I didn't repro the issue myself, but your changes look well-thought-out.