Input lag when using image positioning options

[Rintse]

This issue is a....

[x] Bug
[ ] Other kind of issue (Please describe in detail)

Current Behavior

Very laggy input when using options that change image position.

Expected Behavior

No input lag.

Reproduction Instructions

Grab this image: https://i.imgur.com/SBKjnxm.jpeg (Note, this image has a relatively high pixel count)
Run i3lock -i <image> --scale

Environment

Output of i3lock --version:

i3lock version: 2.13.c.4-34-gd846e7c

Where'd you get i3lock-color from?

[ ] AUR package (which one?)
[x] Built from source yourself
[ ] Other (Please describe in detail)

Seems to be different to #45.
All options that change the placement of the image cause similar lag (-t, -C, -F, -M, -L).

[Raymo111]

What's the full command that you used to launch i3lock-color and get lag with?

[Rintse]

I made sure the issue was present with a minimal cfg.
The command in the original post (i3lock -i <image> --scale) already causes lag.

[Raymo111]

On every keystroke everything is rerendered, so if the image is large and requires complex resizing/moving, it will be really slow. I'll leave this issue open as a "make rerendering more efficient", but at present I don't have a good fix for this. Related to #159 btw.