Fix Linux Build

[cmrschwarz]

This PR allows the pathfinder workspace to compile and pass all tests on Linux again.

I find it very helpful to be able to run cargo test --all in a workspace just to check if I broke anything.

There were three main issues preventing this:

1. The canvas_metal_minimal example expects to link against metal specific APIs:
   I disabled this example on non-apple platforms in a way that's similar to how other projects like rust-skia do this.

2. The android demo had some minor inconsistencies (seems to not have been compiled in a while ... ;)),
   so I fixed those.

3. The Magic Leap demo is pretty fundamentally broken. It uses nonexistant APIs and expects to call into many now private internals.
   I decided to comment out it's member entry in the root workspace.
   Since the hardware it was written for seems to be no longer usable, even for preexisting owners, I was hoping this would be an acceptable tradeoff. It certainly does not seem to make things worse.

With these 3 small changes you can now cargo test --all and cargo build --all the project on Linux again.

[jdm]

These are all reasonable choices. Thanks!

[cmrschwarz]

Awesome, thanks <3.

I know this project has went through a bit of a rough patch, but I still find it very useful even in its current state.

Is there any chance we could get the pathfinder_webgl and maybe even pathfinder (pontuslaestadius/pathtracer#41) crates released on crates io @jdm?

I would be happy to help out with any of the necessary work.