Adjust TCanvas::SetCanvasImp for web case

When web implementation assigned from outside,
several internal fields has to be reset -like fCanvasID or fPixmapID.
Otheriwse canvas cannot be normally displayed and updated.

Extra fields only set when web canvas implementation assigned.

Author: linev

graf2d/gpad/inc/TCanvas.h

@@ -202,7 +202,7 @@ friend class TInterpreter;
    void              SetGrayscale(Bool_t set = kTRUE); // *TOGGLE* *GETTER=IsGrayscale
    void              SetWindowPosition(Int_t x, Int_t y);
    void              SetWindowSize(UInt_t ww, UInt_t wh);
-   void              SetCanvasImp(TCanvasImp *i) { fCanvasImp = i; }
+   void              SetCanvasImp(TCanvasImp *i);
    void              SetCanvasSize(UInt_t ww, UInt_t wh) override; // *MENU*
    void              SetHighLightColor(Color_t col) { fHighLightColor = col; }
    void              SetSelected(TObject *obj) override;

graf2d/gpad/src/TCanvas.cxx

@@ -2156,6 +2156,30 @@ void TCanvas::SetWindowSize(UInt_t ww, UInt_t wh)
       fCanvasImp->SetWindowSize(ww, wh);
 }
 
+////////////////////////////////////////////////////////////////////////////////
+/// Set canvas implementation
+/// If web-based implementation provided, some internal fields also initialized
+
+void TCanvas::SetCanvasImp(TCanvasImp *imp)
+{
+   Bool_t was_web = IsWeb();
+
+   fCanvasImp = imp;
+
+   if (!was_web && IsWeb()) {
+      fCanvasID = fCanvasImp->InitWindow();
+      fPixmapID = 0;
+      fMother = this;
+      if (!fCw) fCw = 800;
+      if (!fCh) fCh = 600;
+   } else if (was_web && !imp) {
+      fCanvasID = -1;
+      fPixmapID = -1;
+      fMother = nullptr;
+      fCw = fCh = 0;
+   }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Set the canvas scale in centimeters.
 ///