set transparent background per render call & screenshot improvements

src/mol-canvas3d/canvas3d.ts

src/mol-canvas3d/passes/draw.ts

src/mol-canvas3d/passes/image.ts

src/mol-canvas3d/passes/multi-sample.ts

src/mol-canvas3d/passes/pick.ts

src/mol-gl/renderer.ts

src/mol-plugin-ui/skin/base/components/controls.scss

src/mol-plugin-ui/viewport/screenshot.tsx

src/mol-plugin-ui/viewport/simple-settings.tsx

src/mol-plugin/util/viewport-screenshot.ts