(This commit also modifies UI)
Changes groups to their own independent type, "group". This allows them
to be used like other regular types, and allows the ability to reference
groups in multiple scenes. Before, a group would always be linked to
the scene it was in. This made it cumbersome for users to modify groups
if they had a similar group in multiple scenes (they would have to
modify each group in each scene). Making groups like other source types
makes more sense to solve this issue so they can be referenced in
multiple scenes at once. This also removes a significant amount of
group-specific handling code required for implementing groups in the
front-end.
One limitation however: due to the way sub-items of groups are
seamlessly modifiable and sortable as part of the whole scene, the user
cannot have multiple references to the same group within one scene.
Prevents a potential cross-lock deadlock. The UI thread would lock the
scene's mutex in obs_scene_atomic_update, then the item would lock the
graphics context to create a texture. Meanwhile in the video thread, it
could lock the graphics context in the render loop, then lock the
scene's mutex when rendering. When doing anything graphics-related, the
graphics context is always supposed to be locked before the scene's
mutex is supposed to be locked (it's designed that way), and the
obs_scene_atomic_update would just bypass that.
Currently if you have a scene, "Scene" for example, and you create a new
scene, the dialog will pop up with a default name, but it'll start with
"Scene 1" instead of "Scene 2". Same thing occurs with sources. This
fixes that to start from 2 instead.
