Non-GPU (software) rendering

[dhardy]

Now that Softbuffer exists, we may consider writing a pure software rendering backend for optimal portability. See also #33 for ideas (e.g. using euc for rendering).

Some refactoring would be required:

• possibly move the winit loop handling to kas-core or another crate
• a new shell crate alongside kas-wgpu
• add a wrapper around shell crates enabling automatic fallback?

[dhardy]

WGPU supports a software fallback option. We don't need another. Enable like this:

KAS_BACKENDS=fallback cargo run --example clock

Or via WgpuBuilder::with_wgpu_options