ios es2 fixes.

- added depth buffer to ES2 renderer.
- added fsaa buffer to ES2 renderer.
- ES2 renderer now has the same features as ES1 renderer
- tested that both ES2 and ES1 renderer both behave the same.
- updated a couple examples so they use ofPushMatrix instead of
glPushMatrix which is not supported in ES2.
Merge pull request #2234 from julapy/ios-es2-fixes

ios es2 fixes.