Add bilinear filtering options to sampling processes

[RobProductions]

We would need less post-smoothing iterations if the terrain heightmap was able to sample from all neighbor pixels instead of just the closest one to the sample position. For ease of use, this could maybe be an extension function or method of the VTRangeGrid. Then nodes like the translation node could sample a precise location instead of directly asking for a pixel and the RangeGrid will handle different filtering cases.