acq stream: for AR static streams, select the top left point by default

We used to pick a random point, which apparently gave a "messy" feeling to some users.
Now it's always the top left point
backend: better error messages when trying to assign a non-existing property

better check whether the VA exists or not, and warn if not.
util img: add rescale function

should work, but barely tested, and not really fast. Need unit test cases.

Also, we should add a rescale_fast which uses order 0.
acq stream: limit input size of ar polar projection

large input sizes (> 1500**2) can consume so much memory (and computation time) that it's not fun.

Hopefully we can find a nicer way to do degraded projection.
gui overlay: don't crash if AR has only one point

of course the distance between points is diffcult to compute....