BUG: enable return value optimzation for VS debug mode

The GetPixel methods require return value optimization to disable a
copy constructor so that the return and used VariableLengthVectors are
internally a reference to the VectorImage's data.

This patch resolve outstanding Interpolator test failures on Visual
Studio debug builds.

Change-Id: I188f589abdcbb230a8217f8850550b6bd50aceed
Merge topic 'VectorImageGetPixelReturnValueOptimization'

38a2f1a BUG: enable return value optimzation for VS debug mode