Like `RegionDistance`, but negative for points inside a solid region.
SignedRegionDistance[reg, p]
SignedRegionDistance[reg]
SignedRegionDistance[Disk[{0, 0}, 1], {3, 0}]
→ 2SignedRegionDistance[Disk[{0, 0}, 1], {0, 0}]
→ -1SignedRegionDistance[Rectangle[{0, 0}, {4, 2}], {1, 1}]
→ -1