Different approach to multiple cameras support. Camera.Main is set to the camera that has the "maincamera" tag
Set default camera priority to 1
Give Camera prefab the "maincamera" tag by default
Add "maincamera" tag to default editor scene
Editor viewport camera shouldn't really care about render tags
Render game viewport from tools if game isn't playing
Clean up GameMenuDll.OnRender
Docs
Use IsMainCamera bool instead of a tag, documented all CameraComponent properties
Don't continuously set Camera.Main