video: fix crash on macOS 10.12

Change-Id: I5e9df28a002be8aa65350b7c2e39663e76f44f98

src/views/CallMTKView.mm

@@ -139,6 +139,7 @@ struct Uniforms {
     [commandEncoder endEncoding];
     [commandBuffer presentDrawable:self.currentDrawable];
     [commandBuffer commit];
+    [self draw];
 }
 
 bool frameDisplayed = false;