Fixed ToolWindow zoom when in Pan mode

[AleksandarDev]

Fixed the issue when using ZoomWindow in Pan mode
Issue #36

The issue is that, when the ActiveTool is Pan but we decided to apply ToolWindow zoom transform (because Shift modifier is active), _dragEndPoint was not set in MouseMove method (because we are in Pan mode).

To fix the problem, mouse event positions e.X, e.Y are used as end point (like in other tools) when mouse click is released.

Do not merge this pull request before #40 because it contains both changes
(my bad, previous PR was on master branch instead of a new branch)

[AleksandarDev]

The new line (as in PR)

var zoomWindowEndPoint = new PointF(e.X, e.Y);

Could be (as _dragEndPoint is not zero when in active tool is ZoomWindow)

var zoomWindowEndPoint = _activeTool == Tools.ZoomWindow ? _dragEndPoint : new PointF(e.X, e.Y);

But I tested both cases (Pan and ToolWindow as active tools) and they behave as expected - same. So the first option should be good enough.