Update FormSet to not recompute each scalling all the time

[jecisc]

Having a scaling that is different of 1 can slow down a lot the image. For example, if we set a background in the image then the perf will be horrible because we will scale the Form of the background all the time and it takes time.

I'm proposing to save the forms we are scaling in the FormSet. Like this we only have to scale once.

Fixes #16884

I'd like a review from @Rinzwind

[jecisc]

I changed my way to deal with the problem. I introduced a CachedFormSet that keeps the scaled versions and I left the FormSet as before. Like this, my change only impacts AlphaImageMorph and not all the other forms.

[Rinzwind]

I would suggest to keep the cached forms separate from the forms the CachedFormSet was constructed with, so that the #forms accessor only gives the latter ones.

It might be sufficient to have just one cached form, so #asFormWithExtent: on CachedFormSet could just answer the cached form if its extent is the same as the argument extent, or else would use the inherited implementation to get the form and set it as the new cached form.

[Rinzwind]

Looks OK, though the sends of #asFormOfDepth: in the implementation of #asFormWithExtent: on CachedFormSet seem redundant, and I’m not sure about not having a limit on the size of the ‘cache’ collection.