Render image previews lazily

[VAWVAW]

Profiling the startup of iamb in foot using flamegraph, I noticed that most of the cpu time was spent rendering image previews. This PR changes the rendering to be in the background (not blocking ui responsiveness) and lazily (on message rendering).

I had some bugs if the last message is an image:

• Scrolling with ^E and ^Y doesn't work if the last message is focused
• if only one image is loaded and the rest of the loaded events are reactions, older events don't load

Profiling data

before optimizations:

with lazily computed sixel preview:

and lazily decoded jpg:

[benjajaja]

I had some bugs if the last message is an image:

* Scrolling with ^E and ^Y doesn't work if the last message is focused

* if only one image is loaded and the rest of the loaded events are reactions, older events don't load

@VAWVAW do you mean these were bugs that are fixed, or these are new issues in this PR?

[VAWVAW]

I just did some more testing and it looks like both are bugs that are in the main branch and were neither introduced nor fixed by this PR.

[benjajaja]

Thank you!

Just tested it, works smooth as butter.