Improve rendering

[aliabid94]

Several improvements to gr.render, including:

1. Animation to show rendering
   

2. Memory cleanup in frontend, removing elements from previous render

3. Allow rendering in a row

4. Respond to last trigger, via trigger_mode="always_last"

[abidlabs]

Tested and this fixed all the issues that I had found previously. LGTM

[freddyaboulton]

Rendering animation looks great. Two comments beyond what I left on slack:

• I think it should be possible to disable the progress animation for render. It can be distracting for quick renders. Especially if render is triggered based on small changes to a state variable.

• I think it should be possible to not queue render events. If it's a quick UI update, I don't see the benefit of queueing.

[pngwn]

I've only tested a little but everything mentioned in the description seems to work well and the frontend changes look good to me! Thanks for fixing these issues @aliabid94!

[pngwn]

Regarding rapid updates, a common approach is to add a threshold of a few hundred MS before starting any animations, that could work here (but is also something we could do in a follow up)

[aliabid94]

I think it should be possible to disable the progress animation for render. It can be distracting for quick renders. Especially if render is triggered based on small changes to a state variable.

Okay I made this change for now: Previously, the animation would be alternating from 100% opacity to 50% and back to 100%, over 2s infinitely. Now it starts at 0%, goes to 100% over 1s, and then 100% opacity to 50% and back to 100%, over 2s infinitely. Because it now starts at 0%, quick renders don't show any visual change.

I think it should be possible to not queue render events. If it's a quick UI update, I don't see the benefit of queueing.

Done.