winemac: Fix failure to render OpenGL on macOS 10.14 (Mojave).

For programs linked against the macOS 10.14 SDK, Mojave makes all view
hierarchies layer-backed.  For views to which OpenGL contexts have been
attached this caused a regression where they sometimes failed to render and
just remain black.  Updating the OpenGL context after the framework has
assigned a layer to our view works around the problem.  Thanks to Elviss
Strazdins on Stack Overflow for the solution
<https://stackoverflow.com/a/52938517/1312143>.

Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/winemac.drv/cocoa_window.m

@@ -747,6 +747,12 @@ static CVReturn WineDisplayLinkCallback(CVDisplayLinkRef displayLink, const CVTi
         [super willRemoveSubview:subview];
     }
 
+    - (void) setLayer:(CALayer*)newLayer
+    {
+        [super setLayer:newLayer];
+        [self updateGLContexts];
+    }
+
     /*
      * ---------- NSTextInputClient methods ----------
      */