Scratchpad geometry problems.

## I'm submitting a…

[x] Bug

[x] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior

Scratchpad size defined with relative units breaks when using multiple monitors.

My actual configuration is:
```
for_window [instance="__scratchpad"] move scratchpad, resize set 100 ppt 80 ppt, move position 0 ppt 0 ppt, move scratchpad
exec uxterm -name __scratchpad
```
Which seems to work more or less fine in a single monitor, but it is still a little hacky. So as it is a kind of hack I am not sure is this is a bug of the actual implementation or a missing feature to do it propperly.

## Desired Behavior

## Environment

Output of `i3 --moreversion 2>&-`:

i3 version:


Binary i3 version:  4.17.1 (2019-08-30) © 2009 Michael Stapelberg and contributors

(Getting version from running i3, press ctrl-c to abort…)

Running i3 version: 4.17.1 (2019-08-30) (pid 232938)

Loaded i3 config: /home/ergo/.config/i3/config (Last modified: Tue 11 Feb 2020 19:58:28 CET, 703 seconds ago)

The i3 binary you just called: /usr/bin/i3

The i3 binary you are running: i3

 

- Linux Distribution & Version: Arch Linux

- Are you using a compositor (e.g., xcompmgr or compton): No