Enable option to not switch to dedicated GPU on mac also with an integrated GPU

[tomfogg]

Adding this flag allows an app built with glutin to run without triggering a switch to the dedicated GPU on a mac (eg on a macbook with low power intel GPU and high power dedicated GPU).

This is mainly for this issue on the alacritty project

From testing, the app also needs to set NSSupportsAutomaticGraphicsSwitching to true in the Info.plist in its app bundle to stop the GPU switch. That means this flag is probably OK to add by default since app behaviour won't change without also having that Info.plist setting.

There are some details about multiple GPU support in this technical note

[tomaka]

Thanks for the PR.

As usual I'm not familiar enough with MacOS to have an opinion on this.
I'll merge in a few days if nobody is opposed to it.

[trishume]

I've looked into this same issue and this is the correct and necessary thing to do. It shouldn't break anything existing since you also need the Info.plist key for it to actually do anything, and even with that key most things should just work.

This is also pretty much a prerequisite for nice GUI frameworks on OSX. Nobody wants their non-intensive apps switching to the high-power GPU.

[tomaka]

Err I totally forgot about this PR.

[tomaka]

Thanks for the insight @trishume

[lilianmoraru]

Seems to fail the CI on macOS(Rust nightly): https://travis-ci.org/tomaka/glutin/builds/251043521

[trishume]

@lilianmoraru the CI breakage isn't related to this PR, it seems to be the cocoa crate that broke.