Update PencilTool.cs

The inserted code will now enable the user to make lines from _startPoint to _endPoint while holding Shift and Left Clicking using the 'Pencil Tool', much like in Photoshop and GIMP.
Update PencilTool.cs
Update PencilTool.cs
Merge pull request #550 from SvardXI/patch-3

Update PencilTool.cs