Ability to cancel request in requestTransform for overzooming purposes

[dee-cf]

Motivation

I am trying to implement overzooming so that I don't need to load vector tile source for each zoom level but just for some.
It would be great to be able to parse zoom levels for which I need to get data from server and for which I can cancel a request.

For example, lets say my tiles extent is such that I can overzoom 3 levels. that means, if I want some data from the source to show on zoom levels 15-21, I could:

15 > load
16, 17 > cancel
18 > load
19,21 > cancel

this way I could save 4 requests per source.

Design Alternatives

The way we can do it now is to have separate layer per zoom level range each with its corresponding source:
15-17 > layer15-17 > source15
18-21 > layer18-21 > source18

alternative approach might be to be able to define an array of zoom levels (within minZoom-maxZoom range) for which source should be loaded. This would be an optional parameter on the source.

Design

The advantage of being able to cancel/skip the request based on zoom level is simplification on layers side (single layer instead of many): we keep single layer with single source and introduce stepped requests based on overzooming strategy.