Fixed bug in GetScalarRange / ComputeScalarRange.

vtkStructuredGrid was overwriting the wrong method (GetScalarRange
instead of ComputeScalarRange) and was updating the wrong MTime
(ComputeTime instead of ScalarRangeComputeTime). Fixed.

Change-Id: I4d582645d6231fe6ebdf7c4b0a33c579833440f8
Fixed thread safety problem.

Two classes were always copying bounds information even if MTime
was older than ComputeTime. This automatically made GetBounds()
a thread problem. Furthermore, they were both always updating
ComputeTime which is both a potential race condition and a bug.

Change-Id: I91f96c0f435119568e7cd5ddbe1a555bc05b00d6
Merge topic 'range-bounds-bug-fixes' into master

afcd8d9c Fixed thread safety problem.
98e39154 Fixed bug in GetScalarRange / ComputeScalarRange.