Add support for resolving multisampled buffers

Some systems are picky about blitting multisampled buffers
So add an option to turn on resolving prior to the final
blit. emscripten sets this by default to true.

Rendering/OpenGL2/vtkOpenGLRenderWindow.cxx

@@ -166,6 +166,7 @@ const char* vtkOpenGLRenderWindow::GetRenderingBackend()
 
 //------------------------------------------------------------------------------
 vtkOpenGLRenderWindow::vtkOpenGLRenderWindow()
+  : BlitRequiresResolve(false)
 {
   this->State = vtkOpenGLState::New();
 
@@ -1015,6 +1016,23 @@ void vtkOpenGLRenderWindow::Frame()
   {
     this->GetState()->PushFramebufferBindings();
     this->OffScreenFramebuffer->Bind(GL_READ_FRAMEBUFFER);
+    int* fbsize = this->OffScreenFramebuffer->GetLastSize();
+
+    // recall Blit upper right corner is exclusive of the range
+    const int srcExtents[4] = { 0, fbsize[0], 0, fbsize[1] };
+
+    if (this->MultiSamples && this->BlitRequiresResolve)
+    {
+      this->GetState()->vtkglViewport(0, 0, fbsize[0], fbsize[1]);
+      this->GetState()->vtkglScissor(0, 0, fbsize[0], fbsize[1]);
+
+      this->ResolveFramebuffer->Bind(GL_DRAW_FRAMEBUFFER);
+      vtkOpenGLFramebufferObject::Blit(srcExtents, srcExtents, GL_COLOR_BUFFER_BIT, GL_NEAREST);
+      this->ResolveFramebuffer->Bind(GL_READ_FRAMEBUFFER);
+    }
+
+    this->GetState()->vtkglViewport(0, 0, this->Size[0], this->Size[1]);
+    this->GetState()->vtkglScissor(0, 0, this->Size[0], this->Size[1]);
 
     this->GetState()->vtkglBindFramebuffer(GL_DRAW_FRAMEBUFFER, this->DefaultFrameBufferId);
     if (this->StereoRender && this->StereoType == VTK_STEREO_CRYSTAL_EYES)
@@ -1026,12 +1044,7 @@ void vtkOpenGLRenderWindow::Frame()
       this->GetState()->vtkglDrawBuffer(this->GetBackLeftBuffer());
     }
 
-    int* fbsize = this->OffScreenFramebuffer->GetLastSize();
-    // recall Blit upper right corner is exclusive of the range
-    const int srcExtents[4] = { 0, fbsize[0], 0, fbsize[1] };
     const int destExtents[4] = { 0, this->Size[0], 0, this->Size[1] };
-    this->GetState()->vtkglViewport(0, 0, this->Size[0], this->Size[1]);
-    this->GetState()->vtkglScissor(0, 0, this->Size[0], this->Size[1]);
     vtkOpenGLFramebufferObject::Blit(srcExtents, destExtents, GL_COLOR_BUFFER_BIT, GL_LINEAR);
     this->GetState()->PopFramebufferBindings();
   }

Rendering/OpenGL2/vtkOpenGLRenderWindow.h

@@ -439,6 +439,11 @@ public:
    */
   void ReleaseGraphicsResources(vtkWindow*) override;
 
+  // set this to true if the destination framebuffer requires a multisamped
+  // buffer to be resolved prior to blitting
+  vtkSetMacro(BlitRequiresResolve, bool);
+  vtkGetMacro(BlitRequiresResolve, bool);
+
 protected:
   vtkOpenGLRenderWindow();
   ~vtkOpenGLRenderWindow() override;
@@ -524,6 +529,11 @@ protected:
 
   int ScreenSize[2];
 
+  // set this to true if the destination framebuffer
+  // requires a multisamped buffer to be resolved
+  // prior to blitting
+  bool BlitRequiresResolve;
+
 private:
   vtkOpenGLRenderWindow(const vtkOpenGLRenderWindow&) = delete;
   void operator=(const vtkOpenGLRenderWindow&) = delete;

Rendering/OpenGL2/vtkSDL2OpenGLRenderWindow.cxx

@@ -42,6 +42,11 @@ vtkSDL2OpenGLRenderWindow::vtkSDL2OpenGLRenderWindow()
   this->SetWindowName(DEFAULT_BASE_WINDOW_NAME.c_str());
   this->SetStencilCapable(1);
 
+  // webgl is picky about multisampled blits
+#ifdef __EMSCRIPTEN__
+  this->BlitRequiresResolve = true;
+#endif
+
   // set position to -1 to let SDL place the window
   // SetPosition will still work. Defaults of 0,0 result
   // in the window title bar being off screen.