Gathers the normal information into sourceRenderTexture.
This function collects the terrain mesh normalmap data from all Terrain tiles in the PaintContext into sourceRenderTexture.
This function is called internally by TerrainPaintUtility.CollectNormals.
Important: There is no corresponding ScatterNormals function, because the normals are not stored, but calculated from the heightmap.
Additional resources: PaintContext, PaintContext.GatherHeightmap