Update BrushTool.cs

Much like the revision I made to the 'Pencil Tool', this revision enables the user to make lines with the 'Brush Tool' from _startPoint to _endPoint by holding Shift and Left Clicking after making their 'Start Point'. User should be mindful of potential memory-based errors or lag when using larger brush sizes. Again, lines will stay within the bound selection box should a selection be active.