Add RFC: Reusable canvas view frontend API

text/0065-reusable-canvas-view-api.md

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+# Summary
+
+API similar to browser panels API to expose the internal implementation of an OBS frontend editable canvas for re-use by developers.
+
+# Motivation
+
+This should enable consistency in plugins which create additional canvases in OBS, and prevent OBS code from being duplicated in this type of plugins.
+
+# Drawbacks
+
+The drawbacks are an additional API to maintain.
+
+# Additional Information
+
+## API interface
+
+```c++
+class QOBSEditableCanvasView: public QWidget {
+public:
+    // When currentScene == nullptr, no scene editing facilities are present
+    //
+    // The class will add a reference to the currentScene which is passed in, and
+    // release previously held reference when switching to a different currentScene.
+    //
+    virtual void setOBSRenderCallbackAndEditableScene(render_callback_t* renderCallback, obs_scene_t* currentScene = nullptr) = 0;
+
+    // Release references to canvas and scene.
+    virtual void clearOBSRenderCallbackAndEditableScene() = 0;
+};
+
+QOBSEditableCanvasView* obs_frontend_create_editable_canvas_view(render_callback_t* renderCallback, obs_scene_t* currentScene = nullptr);
+```
+
+## External render callback
+
+We need an external render callback in order to support rendering different OBS objects to the canvas view surface (sources, canvases, views, etc.)
+
+## Usage
+
+When a component wishing to display an editable canvas initializes, it will call `obs_frontend_create_editable_canvas_view`
+to obtain an instance pointer of `QOBSEditableCanvasView`.
+
+From that point on, it will interact with the QWidget-based class as with any other QWidget, with the addition of specialized
+methods for specifying current canvas & editable scene.
+
+## Changes
+
+This will require changes to `OBSProjector` (and maybe `OBSQTDisplay`) to support additional methods, and calling an external render callback.