Patch to support GLFW_CURSOR_HIDDEN on Mac OS

So, I'd meant to do this quite a while ago and things happened. Anyway, there was actually a bit of a hangup while working on it, mostly because if you tabbed out of a captured window and back in, the cursor position wasn't reset, so I sort of fixed that. I say sort of because it's hard to say, especially when input.c for a while wasn't centering the cursor either. Commit message for 1f148f2bd6 should explain most of that.

Aside from that, the `addCursorRect:cursor:` method mentioned on IRC works well for GLFW_CURSOR_HIDDEN (and should work for GLFW_CURSOR_CAPTURED as well, though the above issue with the cursor position was a problem). At any rate, I axed all calls to NSCursor's hide/unhide method because way too many things could go wrong using them in a multi-window scenario.

Anyhow, there you go, about a month and a half later.