Limit the number of features rendered per layer

Nobody has claimed this yet.

## Motivation

The use case is that I use a symbol layer linked to a source containing a bunch of features. Instead of rendering them all in the viewport, I would like to render up to X of them, counted after collision detection occurred. They are already sorted by rank of importance so I assume that using a counter at rendering level would be sufficient to stop rendering when the counter reach the maximum, still having somehow to deal with collisions that could make some features not appear.

## Current implementation

At the moment I am able to achieve more or less this goal. Whenever the map moves, I loop over the features of the source visible in the viewport, increment a counter and when I reach the limit I want, I apply a property to the remaining features, this property I use in my layer setup to define opacity. That works but is not ideal for two reasons:
1. this occurs as a second round of computation, so it frequently happens that some features appear (from mapbox layer rendering) and quickly disappear (after my own computation)
2. If I loop over all the features of the source, I am not able to detect which of them have been hidden because of a collision; is there a way to get this info at a feature level? Instead I need to loop over the list returned by `queryRenderedFeature`. But if do so, and I hide a feature responsible of a collision because it is out of the limit, then the collided features will show up again...
In order to prevent features from blinking I also tend to not hide a feature that is already visible (ie previous round did not count it out and it did not go out of the viewport during this round).

## Proper implementation

Do you know if there would be better approaches to achieve the desired behaviour, specifically at a lower level? I had a look at the way symbol layers are rendered in the code of mapbox-gl-js but I am not sure yet where collision applies. Also I am wondering if no one requested such a feature before, but I could not find any reference. Thank you in advance!

Start by tracing symbol-layer rendering and collision detection in mapbox-gl-js, including how queryRenderedFeature exposes rendered features. The issue needs an agreed lower-level limit counted after collisions, defined behavior for collided features, and tests covering the limit without blinking.

Written by the indexing model from the issue text.