driver/gldriver: adjust cocoa flavor for gl.Context

Still need to change the X11 implementation. (A separate CL?)

Change-Id: Idf4febe691d3bbba2a0dab27f7a92b2d4ce61db3
Reviewed-on: https://go-review.googlesource.com/14910
Reviewed-by: Nigel Tao <nigeltao@golang.org>
5 files changed