tell our grcontext to abandon its backend

src/views/mac/SkNSView.mm

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #import "SkNSView.h"
 #include "SkCanvas.h"
 #include "SkSurface.h"
@@ skipping 368 matching lines @@
     //it must NOT be deferred or should have been on screen at least once)
     if ([fGLContext view] != self && nil != self.window) {
         [fGLContext setView:self];
     }
 }
 - (bool)attach:(SkOSWindow::SkBackEndTypes)attachType
         withMSAASampleCount:(int) sampleCount
         andGetInfo:(SkOSWindow::AttachmentInfo*) info {
     if (nil == fGLContext) {
         CGLContextObj ctx = createGLContext(sampleCount);
+        SkASSERT(ctx);
         fGLContext = [[NSOpenGLContext alloc] initWithCGLContextObj:ctx];
         CGLReleaseContext(ctx);
         if (NULL == fGLContext) {
             return false;
         }
         [fGLContext setView:self];
     }
 
     [fGLContext makeCurrentContext];
     CGLPixelFormatObj format = CGLGetPixelFormat((CGLContextObj)[fGLContext CGLContextObj]);
@@ skipping 14 matching lines @@
     [fGLContext release];
     fGLContext = nil;
 }
 
 - (void)present {
     if (nil != fGLContext) {
         [fGLContext flushBuffer];
     }
 }
 @end