[gpad] ensure pad painter is created for the canvas

When canvas is initialized one also has to ensure that
painter object is exists. It used to clear drawing area before painting.

Author: linev

graf2d/gpad/src/TCanvas.cxx

@@ -619,14 +619,16 @@ void TCanvas::Build()
       SetBatch(kTRUE);
    } else {
       //normal mode with a screen window
-      // Set default physical canvas attributes
-      fPainter->SelectDrawable(fCanvasID);
-      fPainter->SetAttFill({1, 1001});    //Set color index for fill area
-      fPainter->SetAttLine({1, 1, 1});    //Set color index for lines
-      fPainter->SetAttMarker({1, 1, 1});  //Set color index for markers
-      // fPainter->SetAttText({22, 0., 1, 42, 12}); //Set color index for text
-      // Clear workstation
-      fPainter->ClearWindow(fCanvasID);
+      if (auto pp = GetCanvasPainter()) {
+         // Set default physical canvas attributes
+         pp->SelectDrawable(fCanvasID);
+         pp->SetAttFill({1, 1001});    //Set color index for fill area
+         pp->SetAttLine({1, 1, 1});    //Set color index for lines
+         pp->SetAttMarker({1, 1, 1});  //Set color index for markers
+         // pp->SetAttText({22, 0., 1, 42, 12}); //Set color index for text
+         // Clear workstation
+         pp->ClearWindow(fCanvasID);
+      }
 
       // Set Double Buffer on by default
       SetDoubleBuffer(1);
@@ -662,8 +664,8 @@ void TCanvas::Build()
       SetPad(0, 0, 1, 1);
       Range(0, 0, 1, 1);   //pad range is set by default to [0,1] in x and y
 
-      TVirtualPadPainter *vpp = GetCanvasPainter();
-      if (vpp) vpp->SelectDrawable(fPixmapID);//gVirtualX->SelectPixmap(fPixmapID);    //pixmap must be selected
+      if (auto pp = GetCanvasPainter())
+         pp->SelectDrawable(fPixmapID);      //pixmap must be selected
       PaintBorder(GetFillColor(), kTRUE);    //paint background
    }