Allow VR3-style rendering overlaid on standard editing nodes.

[tbpassin]

This enhancement would allow Leo to optionally render non-text nodes in the same place as the current textual display. Some other software such as Trillium do this, but currently Leo can only render such nodes in a separate frame or tab, as the viewrendered and viewrendered3 plugins do.

One way to do this might be to have VR/VR3 render to a new target that overlays the text node frame rather to a separate frame as they currently do.

Another approach would be to have nodes display in a tabbed widget rather than an editing widget. If the users chooses to display a rendering, the tabbed widget would switch to the rendering tab.

With either approach we could reuse most of the existing VR/VR3 code. Note that VR3 can deal with multiple languages in a single node, which can provide a lot of flexibility.

The freewin plugin already does this kind of dual rendering in a limited way.

[edreamleo]

@tbpassin I have changed the title of this issue, assigned it to you, and given it the 6.7.9 milestone.

I shall be working on #3892, which now applies only to the VR plugin.

[edreamleo]

@tbpassin I see no reason to create a plugin architecture for the VR/VR3 plugins at this time. We can discuss it later if you like.

[edreamleo]

@tbpassin Both the VR and VR3 plugins face a fundamental problem: the is no extra screen space! That's why using a separate window for the VR or VR3 panes is a reasonable thing to consider.

Without creating a separate window, the VR and VR3 plugins can only take space from the body, outline and log panes. It's straightforward to overlay any of these panes without using the nested splitter plugin.

I'll approve any PR you submit that changes only the VR3 plugin.

There's no rush. I've assigned this issue to 6.7.9 only to indicate that I'm willing to consider a PR now.