PIVOT-927:  Seemingly random occurrence of a NullPointerException during
mouse operations causes the UI thread to crash, making the application
unresponsive.

From the stack trace the problem is a null location for the mouse which
is not checked before referencing the location variable.

So, protect all references to the location with a null check first.  Also
make some effort to use the display mouse location if necessary (that is,
if the "mapPointFromAncestor" returns a null location).

This is a merge of revision 1551992 from branches/2.0.x to trunk.


git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1552012 13f79535-47bb-0310-9956-ffa450edef68