Bug 659842 - [topcrash] release the GL context before calling glXDestroyContext - r=karlt

See the glXDestroyContext man page:

  If GLX rendering context ctx is not current to any thread,
  glXDestroyContext  destroys it immediately.  Otherwise, ctx is destroyed
  when it becomes not current to any thread.  In either case, the resource ID
  referenced by ctx is freed immediately.

In other words, if we want glXDestroyContext to have the well-defined semantics
of destroying the context before future X commands take effect, we must first
release the GL context before calling it. We were failing to do that, but we
were destroying the drawable immediately after that call, and as a result, the
context was outliving its underlying drawable. This eventually resulted in
X_GLXMakeCurrent: GLXBadContextTag X errors on subsequent glXMakeCurrent calls.

gfx/thebes/GLContextProviderGLX.cpp

@@ -403,6 +403,11 @@ TRY_AGAIN_NO_SHARING:
     {
         MarkDestroyed();
 
+        // see bug 659842 comment 76
+        bool success = sGLXLibrary.xMakeCurrent(mDisplay, None, nsnull);
+        NS_ABORT_IF_FALSE(success,
+            "glXMakeCurrent failed to release GL context before we call glXDestroyContext!");
+
         sGLXLibrary.xDestroyContext(mDisplay, mContext);
 
         if (mDeleteDrawable) {

toolkit/xre/glxtest.cpp

@@ -212,8 +212,8 @@ static void glxtest()
 
   ///// Clean up. Indeed, the parent process might fail to kill us (e.g. if it doesn't need to check GL info)
   ///// so we might be staying alive for longer than expected, so it's important to consume as little memory as
-  ///// possible.
-  glXDestroyContext(dpy, context);
+  ///// possible. Also we want to check that we're able to do that too without generating X errors.
+  glXMakeCurrent(dpy, None, NULL); // must release the GL context before destroying it
   glXDestroyPixmap(dpy, glxpixmap);
   XFreePixmap(dpy, pixmap);
   XCloseDisplay(dpy);