Helper addition: M (mouse coordinate translation)

Nobody has claimed this yet.

I wanted to see if I could add an interactive element to a dweet, so I decided to attach a mousemove listener to take the mouse coordinate into account. Now, paying for the cost of adding an event listener and whatnot is fair game, but I didn't anticipate how expensive it'd be just to get something canvas-usable out of the mouse event. In the end I dweeted [this little paint demo][1].

[1]: https://www.dwitter.net/d/1577

The annoying boilerplate-y part is the computation of the mouse (x,y) coordinate in relation to canvas coordinates, especially when taking different resolutions (non-fullscreen, fullscreen at different resolutions) into account. It seems reasonable to suggest a helper function to help deal with different resolutions, since that part seems to be meant to be transparent to the dweet (i.e., the canvas is always 1920×1080).

So, I suggest a helper function `M`:

M = (e) => [ e.offsetX/c.offsetWidth *1920,
e.offsetY/c.offsetHeight*1080 ]

The dweeter could then get the mouse coordinates in canvas coordinates via a simple `[X,Y]=M(e)`.

Thoughts? I do realise new helpers should have quite a lot of friction to being added, or we'll just end up abbreviating everything…

Review the proposed M helper and the linked paint demo to understand the expected mouse-coordinate behavior across canvas sizes. Then inspect how helpers are exposed to dweets and determine whether the helper should be accepted; done means the project has an agreed implementation and coverage for the translated coordinates.

Written by the indexing model from the issue text.