shiny/driver/gldriver: new package

Beginning of OS X implementation. Can open windows and receive paint
events timed to the physical display's vsync.

Based on x/mobile/app, but with significant modifications to support
opening more than one window.

Implementation of screen.Texture to follow in a later CL.

Change-Id: I84ffad4d636324fa42799cc3bc0079b8b8607162
Reviewed-on: https://go-review.googlesource.com/12927
Reviewed-by: Nigel Tao <nigeltao@golang.org>
10 files changed