Update: switch GetHeightsFastJobIndirect to use HeightMapData.GetHeight instead of inlined logic
- removed extra 4 madds by using Fast's index calculation
It's slightly slower on 512x512 test (0.9ms -> 1ms) due to Clamp01 calls, but we can afford consistency with GetHeight - will allos us to promote "fast" as "normal" and deduplicate
Tests: ran perf tests, compared asm