feat: add diff screenshot command for pixel-level image comparison

[dannyhw]

Uses pngjs to decode pngs and then compares each pixel with euclidean distance.
Mirrors the agent-browser CLI interface.

Usage:
agent-device diff screenshot --baseline before.png [--out diff.png] [--threshold 0.2]

try it out with

use scroll down for diff

node bin/agent-device.mjs open Settings --platform ios 
sleep 1
node bin/agent-device.mjs screenshot before.png 
node bin/agent-device.mjs scroll down 0.5
node bin/agent-device.mjs diff screenshot --baseline before.png --out diff.png

take two equal screenshots

node bin/agent-device.mjs open Settings --platform ios 
node bin/agent-device.mjs screenshot before.png 
node bin/agent-device.mjs diff screenshot --baseline before.png --out diff.png

Example diff

[thymikee]

odiff is 22,5 MB, can we find a lighter alternative?

[dannyhw]

replaced with a custom solution using just pngjs for decoding pngs

[dannyhw]

updated now to set the time to 9:41 before any screenshots so the time won't be a factor in diffs

[thymikee]

Thank you! This is pretty cool :D

Android diff	iOS diff