OpenGL examples doesn't compile

[SimonWoodburyForget]

Issues:

• the project doesn't have the gl feature flag anymore,
• wgpu-rs doesn't seem to have Instance anymore,

edit: I just found gfx-rs/wgpu#450 which seems to state OpenGL support was deprecated?

[grovesNL]

Support for GL was temporarily removed because of some restrictions in GL that we haven't worked around yet (mostly handling GL contexts/surfaces/swapchain).

gfx-rs/wgpu#450 is the best tracking issue for re-enabling GL support

[zicklag]

Not everything has been fixed yet, but OpenGL support has been re-enabled on master! #606