Slower to start and open when with wgpu

[ids1024]

Built with cargo build --no-default-features, the NewToplevel event seems to happen sooner, and the workspaces view takes longer to show. (I thought the latter problem was due to the time required to create a Vulkan surface; not sure about the slower behavior at start.)

This probably applies to all libcosmic apps that use wgpu.

This switched to wgpu by default for performance, but maybe that's not needed now, at least if the compositor or a subsurface handles rendering the background.

[ids1024]

pop-os/cosmic-comp#827 tracks this issue more broadly. I suppose nothing here is specifically a cosmic-workspaces issue, though Vulkan and such could possibly create fewer registries.