Set test material to lava instead of stone
Update voxel-engine - negate and flip camera vector, ref GH-2

deathcap/voxel-engine@ef8c0eeb875b378e0267f385d5db1011eed072f4 Negate and flip camera vector - partially fixes raycasting, https://github.com/deathcap/voxel-example/issues/2

Commit message generated by https://github.com/deathcap/lmno-cl