macosx_vout: Correctly change context after -lockgl.

And don't unlockgl if lockgl wasn't successful.
(cherry picked from commit 0484b127)

modules/video_output/macosx.m

@@ -419,7 +419,10 @@ static void OpenglSwap(vout_opengl_t *gl)
 - (BOOL)lockgl
 {
     VLCAssertMainThread();
-    CGLError err = CGLLockContext([[self openGLContext] CGLContextObj]);
+    NSOpenGLContext *context = [self openGLContext];
+    CGLError err = CGLLockContext([context CGLContextObj]);
+    if (err == kCGLNoError)
+        [context makeCurrentContext];
     return err == kCGLNoError;
 }
 
@@ -493,8 +496,7 @@ static void OpenglSwap(vout_opengl_t *gl)
         }
     }
 
-    [self lockgl];
-
+    if ([self lockgl]) {
         glViewport((width - x) / 2, (height - y) / 2, x, y);
 
         @synchronized(self) {
@@ -508,6 +510,7 @@ static void OpenglSwap(vout_opengl_t *gl)
         [self unlockgl];
 
         [super reshape];
+    }
 }
 
 /**