Merge pull request #591 from mozilla/pngcrush

src/assets/grid.png

src/assets/hud/bubble_off-hover.png

src/assets/hud/bubble_off.png

src/assets/hud/bubble_on-hover.png

src/assets/hud/bubble_on.png

src/assets/hud/create_object-hover.png

src/assets/hud/create_object.png

src/assets/hud/freeze_off-hover.png

src/assets/hud/freeze_off.png

src/assets/hud/freeze_on-hover.png

src/assets/hud/freeze_on.png

src/assets/hud/mute_off-hover.png

src/assets/hud/mute_off.png

src/assets/hud/mute_on-hover.png

src/assets/hud/mute_on.png

src/assets/hud/spawn_camera-hover.png

src/assets/hud/spawn_camera.png

src/assets/hud/spawn_pen-hover.png

src/assets/hud/spawn_pen.png

src/assets/hud/spawn_photo-hover.png