Minor tweaks

See merge request bilelmoussaoui/gnome-tour!2

assets: proper unified scale

See merge request bilelmoussaoui/gnome-tour!1

- scaled up all assets for them to render sharp
- unified the sizing so they don't jump around
- unified stroke sizes
- optimized with SVGO

original assets to create derivatives are at
https://gitlab.gnome.org/Teams/Design/os-mockups/tree/master/greeter/assets