handle negative isovalues in gpu mc

src/mol-gl/compute/marching-cubes/isosurface.ts

src/mol-gl/shader/marching-cubes/isosurface.frag.ts

src/mol-repr/structure/visual/gaussian-surface-mesh.ts

src/mol-repr/volume/isosurface.ts

src/tests/browser/marching-cubes.ts