More options for querying and changing opacity

[smason]

Similar to #7173 I wanted a nicer way to get control over opacity of windows. These changes makes it easy to get the opacity of the current window, e.g., via:

swaymsg -r opacity get

as well as providing more options for altering it via mul and div subcommands, e.g.,:

swaymsg opacity mul 1.25
swaymsg opacity div 1.25

As per the commit message, human senses tend to work on a log-scale; hence multiplicative changes like this tend to feel smoother than linear changes (i.e. via the existing plus and minus).

This change doesn't expose the opacity in the get_tree output as I didn't need it, but could likely add that in if somebody cares about it.

[emersion]

I don't think we'd want more commands to mutate the opacity. These div/mult operations can easily be done by the IPC client, e.g. in a shell.

I don't think a global "get opacity" command is the way to go. Returning the opacity in the get_tree response would probably make more sense.

All that said not sure we want to expand the opacity stuff at all. It doesn't work with sub-surfaces correctly so maybe we will axe it in a future release.

[smason]

All that said not sure we want to expand the opacity stuff at all. It doesn't work with sub-surfaces correctly

What's wrong with this? Is this why, e.g., menus in firefox become transparent when the whole window does?

so maybe we will axe it in a future release.

Meh, no great preference. I did this because I thought it might me nice to have a transparent video player running over my text-editor in the background and got a bit carried away while configuring. After using it a bit have decided that it doesn't look very good anyway! Ah well, interesting seeing more of the insides of Sway.