Fix crashes when changing toggling fullscreen/windowed mode.

- Only allow such changes when in emulation mode, or a few other common ones (launcher, debugger)
  The idea is here that some dialogs have to be resized/repositioned when such a mode change occurs, and the UI core doesn't currently support that.
- Only render surfaces when they are visible (sounds logical enough, but the code didn't check for it).

src/emucore/FrameBuffer.cxx

@@ -652,6 +652,22 @@ void FrameBuffer::stateChanged(EventHandlerState state)
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 void FrameBuffer::setFullscreen(bool enable)
 {
+  // Switching between fullscreen and windowed modes will invariably mean
+  // that the 'window' resolution changes.  Currently, dialogs are not
+  // able to resize themselves when they are actively being shown
+  // (they would have to be closed and then re-opened, etc).
+  // For now, we simply disallow screen switches in such modes
+  switch(myOSystem.eventHandler().state())
+  {
+    case EventHandlerState::EMULATION:
+    case EventHandlerState::LAUNCHER:
+    case EventHandlerState::DEBUGGER:
+    case EventHandlerState::PAUSE:
+      break; // continue with processing (aka, allow a mode switch)
+    default:
+      return;
+  }
+
   const VideoMode& mode = getSavedVidMode(enable);
   if(setVideoMode(myScreenTitle, mode))
   {

src/gui/Dialog.cxx

@@ -143,18 +143,15 @@ void Dialog::setTitle(const string& title)
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 void Dialog::center()
 {
-  if(_surface)
+  const GUI::Size& screen = instance().frameBuffer().screenSize();
-  {
+  const GUI::Rect& dst = _surface->dstRect();
-    const GUI::Size& screen = instance().frameBuffer().screenSize();
+  _surface->setDstPos((screen.w - dst.width()) >> 1, (screen.h - dst.height()) >> 1);
-    const GUI::Rect& dst = _surface->dstRect();
-    _surface->setDstPos((screen.w - dst.width()) >> 1, (screen.h - dst.height()) >> 1);
-  }
 }
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 bool Dialog::render()
 {
-  if(!_dirty)
+  if(!_dirty || !isVisible())
     return false;
 
   // Draw this dialog

src/gui/Rect.hxx

@@ -110,15 +110,12 @@ struct Rect
   uInt32 top, left;        //!< The point at the top left of the rectangle (part of the rect).
   uInt32 bottom, right;    //!< The point at the bottom right of the rectangle (not part of the rect).
 
-  Rect() : top(0), left(0), bottom(0), right(0) { }
+  Rect() : top(0), left(0), bottom(0), right(0) { assert(valid()); }
-  Rect(const Rect& s) : top(s.top), left(s.left), bottom(s.bottom), right(s.right) { }
+  Rect(const Rect& s) : top(s.top), left(s.left), bottom(s.bottom), right(s.right) { assert(valid()); }
   Rect& operator=(const Rect&) = default;
-  Rect(uInt32 w, uInt32 h) : top(0), left(0), bottom(h), right(w) { }
+  Rect(uInt32 w, uInt32 h) : top(0), left(0), bottom(h), right(w) { assert(valid()); }
-  Rect(const Point& p, uInt32 w, uInt32 h) : top(p.y), left(p.x), bottom(h), right(w) { }
+  Rect(const Point& p, uInt32 w, uInt32 h) : top(p.y), left(p.x), bottom(h), right(w) { assert(valid()); }
-  Rect(uInt32 x1, uInt32 y1, uInt32 x2, uInt32 y2) : top(y1), left(x1), bottom(y2), right(x2)
+  Rect(uInt32 x1, uInt32 y1, uInt32 x2, uInt32 y2) : top(y1), left(x1), bottom(y2), right(x2) { assert(valid()); }
-  {
-    assert(valid());
-  }
 
   uInt32 x() const { return left; }
   uInt32 y() const { return top; }