Rusting 0.1.6

What's Changed

• Timeline: clear frame, insert/delete frame shortcuts, move frame selection by @HalfdanIsaksen in #14
• feat: paint behind with brush by @HalfdanIsaksen in #15
• Straight brush lines, rectangle and ellipse tools with gradient fill by @HalfdanIsaksen in #16

Full Changelog: v0.1.5...v0.1.6