Allow creating pure window context without GL init

[kvark]

When working with gfx_device_gl (maybe glium too?), the GL function pointers are initialized twice

1. On window creation, using gl:: namespace
2. On gfx_device_gl::create(), using the internal gl struct

These seem to be in conflict sometimes (PistonDevelopers/piston-examples#200), not to mention the amount of wasted work, which slows down the start time.

I do realize that it might be simpler to add another init method for gfx_device_gl, but that goes against the safety guarantees of gfx. Besides, that would require all the gfx_device_gl code to use the global gl:: instead of a struct.

Ideally, I'd like it to be a compile time flag for things that implement piston-window. Thus, gfx_cube example would compile glfw_window (or sdl2_window) in a "detached" state that doesn't touch OpenGL and only initializes GLFW (or SDL).

[Potpourri]

Compile time flag sounds nice.

[Potpourri]

I think we should remove GL init from window back-ends. It was added only for opengl_graphics back-end. Now GL init worth move to opengl_graphics, this will be correct.

[kvark]

Oh that would be great!

[bvssvni]

Alternative is to add a flag to WindowSettings of whether to initialize OpenGL or not.

[bvssvni]

@Potpourri Are you on IRC #rust-gamedev?

[bvssvni]

This discussion got mixed up a bit between two issues.

It seems that since Gfx uses a different library for gl, it makes sense with a Cargo feature like @kvark suggested.

[bvssvni]

Ideally I would like to make it explicit in code what is happening. Easier to debug problems.

[bvssvni]

I've tried overriding sdl2_window, removing the OpenGL initialization and measure the build time. No difference. On my machine, it takes a approximately 1 minute to build the gfx_cube example.

cargo clean
cargo build

real    1m6.963s
user    4m2.679s
sys 0m11.315s

[bvssvni]

So far I have not experienced any problems with running Gfx and OpenGL side by side, so this seems to be an issue with 3.1.

[bvssvni]

Todo list for window backends:

• glutin_window
• glfw_window
• sdl2_window