run background images through optipng

`optipng -o7 select-background-*.png`

~2-3k

```bash
osmith osmith-ubuntu | Tue Oct 14 at 16:03:50 | git master
/home/osmith/dev/vui/valence-ui-input ~ du -cb select-background-*.png
3915	select-background-off.png
3774	select-background-on.png
3881	select-background-rtl-off.png
3792	select-background-rtl-on.png
15362	total
```
```bash
osmith osmith-ubuntu | Tue Oct 14 at 16:04:11 | git minify-images
/home/osmith/dev/vui/valence-ui-input ~ du -cb select-background-*.png
3076	select-background-off.png
3080	select-background-on.png
3097	select-background-rtl-off.png
3103	select-background-rtl-on.png
12356	total
```