Drawing and sending paintings in-app

**Is your feature request related to a problem? Please describe.**

Often it's helpful if you can describe something in pictures rather than words.
Here are some situations:

* Wanting to do art and draw a painting.
* Preferring to write text by hand rather than type text.

For example I have a tablet that I can connect to my desktop PC so I could write messages
by hand. Mobile users could do this with their fingers or a touch pen or so.

* Practicing writing systems with a friend.

The possibilities are endless.

Other apps already have this:

* Messages on iOS has Digital Touch which allows you to draw paintings and send them, right in the app. I do believe that there could be many people that regularly use this feature and would miss it if they switch to Element.
* The Wire messenger also has an inbuilt drawing feature.

I think it would be a fun little feature.

**Describe the solution you'd like**

I think it'd be pretty cool if Element for mobile \(so the iOS and Android versions\) could support drawing with your fingers and then you can send the image, right in the app. Desktop/browser could also support it and you would draw using the mouse.

For example on desktop/browser, there could be a pen icon next to the emoji icon in the bottom right and then you click on it and a little canvas pops up, or maybe it could be part of the other options in the menu of the 3-dot button.
Depending on your theme, the canvas background is dark or light \(this is important if you draw at night\). Then you have a few colors to choose from and can draw the painting. Then you can send it.

Optionally, after you send the painting, it could play an animation of your strokes and how you drew but I don't think this is absolutely necessary.
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