v0.8.0: selection subtract, inverted cutouts, package, URP/HDRP samples

• Editing: rectangle splat selection can now subtract from selection while Ctrl key is pressed (thx @JasonDeacutis).
• Editing: cutouts got an option to "invert" their logic, i.e. remove splats inside their area, instead of leaving splats inside their area.
• Project structure has been reorganized into a Unity package org.nesnausk.gaussian-splatting under package folder, and three sample projects under projects folder, one each for Built-in, URP and HDRP rendering pipelines.
• Fix: scripts were not compiling when URP or HDRP was used, lol.
• Fix: splats don't try to render themselves into any material preview under URP.