vtkOsmLayer: refactor std::pow call

Also turn it into a bitshift since it zoomLevel is always an integer,
negative powers don't make logical sense anyways (it's always truncated
to an integer).
Merge pull request #71 from mathstuf/pow-ambiguity

vtkOsmLayer: replace std::pow(2, int) with a bitshift
Merge branch 'master' into databrowser

Conflicts:
	vtkOsmLayer.cxx