Use a transparent color for the transparent example

[stefnotch]

• Tested on all platforms changed
• Created or updated an example program if it would help users understand this functionality

The transparency example used a solid fill color, which seems a tad odd. This fixes this by using a semi-transparent gray. However, we should wait for softbuffer to support transparency. rust-windowing/softbuffer#17
I only have a Windows machine, so I only managed to test the example on Windows.

The example can be tested with cargo run --example transparent

[stefnotch]

I'd love some advice on how to fix the CI.

[madsmtm]

I'd love some advice on how to fix the CI.

Maybe just slap a #[allow(dead_code)] on fill_window?

[kchibisov]

Allowing dead code sounds fine to me.