Window creation examples for SDL and GLFW

[kvark]

Basic examples can be found at https://github.com/icefoxen/gfx/tree/master/examples/triangle-glfw and https://github.com/icefoxen/gfx/tree/master/examples/triangle-sdl

cc @icefoxen

[icefoxen]

See, now I'm going to have to eat my words about "keeping those up to date is trivial", since they're not up to date... XD

[kellpossible]

and an example with glutin too?
edit: nevermind, the just realised the default example uses glutin

[alexheretic]

#1271 provides an sdl window example, run with (cd src/window/sdl && cargo run --example window)

[kvark]

Fixed by #1258 and #1271
Thanks everyone!