Make :winpos measure Y coordinates from top

This is to make :winpos consistent with other ports where Y coordinates
are measured from the top instead of the bottom of the screen.

src/MacVim/MMVimController.m

@@ -851,6 +851,10 @@ static BOOL isUnsafeMessage(int msgid);
         int x = *((int*)bytes);  bytes += sizeof(int);
         int y = *((int*)bytes);  bytes += sizeof(int);
 
+        // NOTE: Vim measures Y-coordinates from top of screen.
+        NSRect frame = [[[windowController window] screen] frame];
+        y = NSMaxY(frame) - y;
+
         [windowController setTopLeft:NSMakePoint(x,y)];
     } else if (SetTooltipMsgID == msgid) {
         id textView = [[windowController vimView] textView];

src/MacVim/MMWindowController.m

@@ -874,7 +874,10 @@
 
     // NOTE: This method is called when the user drags the window, but not when
     // the top left point changes programmatically.
-    int pos[2] = { (int)topLeft.x, (int)topLeft.y };
+    // NOTE 2: Vim counts Y-coordinates from the top of the screen.
+    int pos[2] = {
+            (int)topLeft.x,
+            (int)(NSMaxY([[decoratedWindow screen] frame]) - topLeft.y) };
     NSData *data = [NSData dataWithBytes:pos length:2*sizeof(int)];
     [vimController sendMessage:SetWindowPositionMsgID data:data];
 }
@@ -1062,7 +1065,10 @@
         // NOTE: The window top left position may change due to the window
         // being moved e.g. when the tabline is shown so we must tell Vim what
         // the new window position is here.
-        int pos[2] = { (int)newTopLeft.x, (int)newTopLeft.y };
+        // NOTE 2: Vim measures Y-coordinates from top of screen.
+        int pos[2] = {
+            (int)newTopLeft.x,
+            (int)(NSMaxY([[decoratedWindow screen] frame]) - newTopLeft.y) };
         NSData *data = [NSData dataWithBytes:pos length:2*sizeof(int)];
         [vimController sendMessage:SetWindowPositionMsgID data:data];
     }