evas-gl_cocoa: migrate to gl_generic infrastructure

evas-gl_cocoa: migrate to gl_generic infrastructure

It has been a long journey, but here we are at last...
The infamous gl_cocoa engine has been migrated to the
gl_generic infrastructure. This should provide great
improvements and hopefully reduce side-channels b0rkage.

Fonts seems better, scrolling is smoother, expedite
does not segfault anymore... I haven't found a
regression with elementary_test, elementary_config,
terminology, expedite.