suppress wgpu warnings by default

[cart]

They are generally unactionable and noisy. They scare users constantly / make them assume they are doing something wrong. Almost without exception they are:

• Warnings about gaps in Naga validation layers. Naga is still pretty new and it can't fully validate shaders. Disabling the validation layers also produces a warning that the validation layers are disabled.
• Warnings about unsupported features / fallbacks (ex: falling back to Fifo vsync when Mailbox isn't supported)

These can still be opted into by setting the RUST_LOG=wgpu=warn environment variable.