Fix issue #5248: don't redo placement for zoom changes of low-pitch m…

[ChrisLoer]

Fixes #5248.

aefb3ee introduced redoPlacement calls in response to any map move, since any movement could potentially affect collision detection with the new pitch-scaling behavior. To prevent incurring extra/unnecessary placement costs, I did a couple things:

• Don't trigger placement at all if none of the transform parameters had changed
• Don't trigger placement if the only thing that had changed was the relative position of the tile (i.e. a pan operation), and the pitch was low (at low pitch, pan operations have very small effects on collision detection).
• Trigger placement at most once per 300 ms: there's no point in maxing out the CPU on the worker threads to deliver placement faster than collisions are actually perceived, and in fact going too quickly can exacerbate some "flicker" problems.

However, I forgot that the transform parameter cameraToCenterDistance would be changing with every zoom change, and that zoom changes didn't previously trigger placement. While zoom changes (and anything else that affects cameraToCenterDistance, like changing the field of view) do affect collision detection, the effects are minimal at low pitch and can be safely ignored.

When the cost of placement is relatively low, this is not a very substantial difference, because the throttling limits placement to once every 300ms anyway. When placement is significantly more expensive (as in #5208), the placement can end up running continuously during zoom operations, which is a waste (we should only have to re-do placement when we cross integer zoom levels).

This PR fixes the oversight and introduces one unit test to verify that redoPlacement is actually being suppressed.

This will hopefully all be superseded soon by #5150.

bench results

The benchmarks don't really exercise the use case this PR addresses, so this is just a sanity check.

benchmark	master 7d452ea	cloer_5248 6e2fbf4
map-load	125 ms	89 ms
style-load	127 ms	132 ms
buffer	1,147 ms	1,128 ms
tile_layout_dds	1,259 ms	1,258 ms
fps	60 fps	60 fps
frame-duration	6.2 ms, 0% > 16ms	6.4 ms, 1% > 16ms
query-point	0.84 ms	0.96 ms
query-box	72.64 ms	66.96 ms
geojson-setdata-small	9 ms	11 ms
geojson-setdata-large	130 ms	148 ms
filter	n/a	n/a

/cc @jfirebaugh @ansis

[jfirebaugh]

Thanks!