Actually run tests on a big endian architecture on CI

[Shnatsel]

instead of just building on PowerPC and calling it a day

workflow definition is cribbed from image-webp

[Shnatsel]

The build fails because dev-dependencies are shared between tests and examples, and one of our examples pulls in glium which in turn pulls in a sprawling dependency tree and ends up missing some headers in cross container.

I guess the best way to work around that is to split the show.rs example into its own crate with its own Cargo.toml and add a separate CI job checking that it still compiles. Any better ideas?

[fintelia]

I'd also be fine just dropping the show example. The majority of it looks like it is about how to use glium to display images rather than anything to do with PNG decoding.

[Shnatsel]

I deleted the glium example and that got powerpc CI working. Still passes, too!