v0.9.0: more editing tools, merge, apply transform, faster GPU sort

Editing (#51):

• Splat editing tools are now the Unity "editor tool context" thing (blob looking icon atop of regular tools toolbar).
• Can now move individual selected splats.
  
• When multiple splat objects are selected, they can be merged into one larger splat cloud.
• Add "Reset" button to revert edits back to asset state.

Export:

• Option to "bake" (i.e. apply) whole object transformation into the exported PLY file. This included proper rotation of Spherical Harmonics data.

Rendering:

• Faster GPU sorting routine, AMD FidelityFX 4-bit radix sort was changed to "reduce-then-scan" 8-bit radix sort (contributed in #82 by @b0nes164, thanks!). On RTX 3080Ti, sorting 6.1 million splats goes from 2.4ms down to 1.1ms.
• Add option to visualize only the spherical harmonics contribution, without regular color.

⚠️ Note: splat asset format has changed, need to re-create the splat assets.