always use glsl300 for webgl2

src/mol-geo/representation/structure/units-visual.ts

src/mol-gl/renderable/direct-volume.ts

src/mol-gl/renderable/lines.ts

src/mol-gl/renderable/mesh.ts

src/mol-gl/renderable/points.ts

src/mol-gl/renderable/schema.ts

src/mol-gl/shader-code.ts

src/mol-gl/shader/mesh.frag

src/mol-gl/webgl/program.ts