Add `WindowSettings::draw_size`

[bvssvni]

See https://www.reddit.com/r/rust_gamedev/comments/3ejcy2/using_piston_how_can_i_configure_a_set_game/

Currently the window back-end decides the resolution of the frame buffer.

[bvssvni]

One problem is that PistonWindow automatically resizes the frame buffer.

Probably need some way to tell whether one should resize. Could be AdvancedWindow::should_resize_draw_buffer.

[bvssvni]

The Iterator is implemented for PistonWindow with the Window constraint, so it needs to be Window::should_resize_draw_buffer.

[bvssvni]

Doesn't seem like Glutin support this.

[bvssvni]

rust-sdl2 doesn't support this either, it seems.

[bvssvni]

It seems that window mode automatically sets the frame buffer. The only way to fix the resolution is to open the window in full screen.

[bvssvni]

It remains to figure out whether PistonWindow should resize the buffer or not.

[bvssvni]

I believe this is a viewport/transformation issue. It might be possible to do this without needing a setting.

[bvssvni]

I haven't managed to find a good solution for this. Reopen if you have better ideas.