Non-threadsafe Carbon API use in aglManage

In the Mac OS version of aglManage, modules/gui/minimal_macosx/voutagl.m, various non-threadsafe Carbon Window Manager API are used in the VOUT_FULLSCREEN_CHANGE case to create a new window and initialize its state. However, this code is sometimes run on threads other than the main thread. At Apple, we are seeing reports of this usage causing a significant number of crashes in Safari. I've attached a sample backtrace.

It's very important that no Carbon Window Manager API be used from any thread other than the main thread. We cannot fix this in Mac OS X; it must be fixed by the VLC project.