Compositor: Replace resolution concept by canvas

This is a code refactor in preparation of supporting canvas
compositing. See {D12466}.

No functional changes, all canvases are at (0,0) position matching
tiled implementation.

Differential Revision: https://developer.blender.org/D12465

source/blender/compositor/operations/COM_ImageOperation.h

@@ -54,8 +54,7 @@ class BaseImageOperation : public MultiThreadedOperation {
   /**
    * Determine the output resolution. The resolution is retrieved from the Renderer
    */
-  void determineResolution(unsigned int resolution[2],
-                           unsigned int preferredResolution[2]) override;
+  void determine_canvas(const rcti &preferred_area, rcti &r_area) override;
 
   virtual ImBuf *getImBuf();