v0.7.0: faster rendering and cutout objects

• Performance: Rendering is faster (#28), e.g. a 6 million splat "bicycle" scene on M1 Mac goes 31.8ms -> 23.6ms. This is still using a traditional "render each splat as a quad" rasterizer, but each quad is smaller and more tightly bounds the splat.
• Editing: In addition to manual selection & deletion of splats, you can also place "Cutout" objects in ellipsoid or box shape (#31). Thanks @hybridherbst for the initial implementation!
• Fix: improved robustness or PLY loading & exporting (#30)
• UX: more recent entries shown in PLY file picker UI widget
• Clarified license (MIT)