Respond to backing properties changes

This fixes a bug which caused the window to be white after moving it
between a Retina and normal display.

src/MacVim/MMBackend.m

@@ -2092,6 +2092,8 @@ static void netbeansReadCallback(CFSocketRef s,
         [self setImState:YES];
     } else if (DeactivatedImMsgID == msgid) {
         [self setImState:NO];
+    } else if (BackingPropertiesChangedMsgID == msgid) {
+        [self redrawScreen];
     } else {
         ASLogWarn(@"Unknown message received (msgid=%d)", msgid);
     }

src/MacVim/MMWindowController.m

@@ -1002,6 +1002,11 @@
     [vimView setFrameSize:[self contentSize]];
 }
 
+- (void)windowDidChangeBackingProperties:(NSNotification *)notification
+{
+    [vimController sendMessage:BackingPropertiesChangedMsgID data:nil];
+}
+
 // This is not an NSWindow delegate method, our custom MMWindow class calls it
 // instead of the usual windowWillUseStandardFrame:defaultFrame:.
 - (IBAction)zoom:(id)sender

src/MacVim/MacVim.h

@@ -192,6 +192,7 @@ enum {
     SetTooltipDelayMsgID,
     GestureMsgID,
     AddToMRUMsgID,
+    BackingPropertiesChangedMsgID,
     LastMsgID   // NOTE: MUST BE LAST MESSAGE IN ENUM!
 };
 

src/MacVim/MacVim.m

@@ -99,6 +99,7 @@ char *MessageStrings[] =
     "SetTooltipDelayMsgID",
     "GestureMsgID",
     "AddToMRUMsgID",
+    "BackingPropertiesChangedMsgID",
     "END OF MESSAGE IDs"     // NOTE: Must be last!
 };