timeVaryingMappedFixedValue uses linear extrapolation if outside triangulation

timevaryingMappedFixedValue creates a triangulation 'under the hood'. It then uses bilinear interpolation for all sample points inside a triangle. For sample points outside all triangles it uses a bilinear interpolation using the nearest triangle. This is generally not ideal and especially very high aspect ratio triangles it gives problems (e.g. interpolation weights (23937.3 -48293 24356.7)

I think we should use zero-gradient outside triangles.