Unity Scriptable Render Loop testbed

NOTE: this is a testbed for a Unity feature that has not shipped yet! The project will not work with any public Unity version, and things in it might and will be broken.

"Scriptable Render Loops" is a potential future Unity feature, think "Command Buffers, take two". We plan to ship the feature, and a new modern built-in rendering loop with it. For now you can look around if you're really curious, but like said above, this is not useful for any public Unity version yet.