World XZ of the splatmap's (0,0) texel — the rect's min corner.
World span (XZ) the splatmap covers.
OptionaldataOptional CPU copies of the weight maps (a = layers 0-3, b = 4-7).
When provided, TerrainSystem.splatWeightsAt works — foliage
scattering and the editor's paint tools read (and edit) these.
Maps authored splat weight textures onto world space.