Context::new_context's shareable_with_windowed_contexts parameter only signficiant on Linux

[Osspial]

On non-Linux platforms, the parameter is either ignored or crashes the program if set to true. There doesn't seem to be a good reason for this to be a cross-platform parameter, much less one that invades the main API for creating headless contexts. Instead, it should be a parameter in a linux ContextBuilderExt trait that is named for what it actually does - choosing between a window-manager-backed context or an OSMesa context.

[goddessfreya]

Hey @Osspial, this was a great idea which I'd never thought about. Should've cc'd me so that I'd done it earlier. :)
Been added to my growing PR, right here: #1058

[goddessfreya]

Fixed in #1101.