vtkOsmLayer: replace std::pow(2, int) with a bitshift

std::pow is being reported as ambiguous on Windows, but since zoomLevel
is an integer and the base is always 2, a bitshift can be used instead
(not to mention that std::pow is called lots of times and should
probably be cached in a local variable instead).
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See https://www.kitware.com/CDash/viewBuildError.php?buildid=528216