keep track of gl state to reduce state changes

src/mol-gl/compute/histogram-pyramid/reduction.ts

src/mol-gl/compute/marching-cubes/active-voxels.ts

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

src/mol-gl/renderer.ts

src/mol-gl/webgl/context.ts

src/mol-gl/webgl/program.ts

src/mol-math/geometry/gaussian-density/gpu.ts

src/tests/browser/render-structure.ts