image addon works

[jerch]

The image addon still has some lose ends:

• allow overprinting with blending (needed for correct handling of transparent pixel overwrites)
• better story for backgroundSelect 0 vs. 2
• change final cursor position to VT340 mode as default
• find some working serialize solution (prolly needs re-encoding with some PNG encoder)
• investigate in worker based drawing (OffscreenCanvas, createImageBitmap)
• better image protocol with PNG/JPG support (iTerm2 support?)
• better addon ctor settings handling in demo page
• direct nodejs support without browser frontend (needs drawing and export shims)
• do some sort of pixel output tests on terminal buffer cells