CrossTileSymbolIndex is copying itself on every frame

[ChrisLoer]

While trying to follow up on an Android perf issue I noticed that the CrossTileSymbolIndex was showing up way too prominently in the profiler (~10% of CPU time on render thread). It turns out we started copying the entire index on every frame in an otherwise minor back-port of changes from GL JS (feab36a#diff-8f7f3c3dea918c3c149a37c11add87e8)

The problem is that the "auto" here makes a copy instead of taking a reference to the result of indexes[tileID.overscaledZ].

mapbox-gl-native/src/mbgl/text/cross_tile_symbol_index.cpp

Line 65 in b820ca4

auto thisZoomIndexes = indexes[tileID.overscaledZ];

Changing that to an "auto&" cuts the time spent in CrossTileSymbolIndex::addLayer by about 90%.

The copy doesn't break anything because if we actually need to make a change we explicitly add a new index for the layer:

mapbox-gl-native/src/mbgl/text/cross_tile_symbol_index.cpp

Lines 110 to 111 in b820ca4

	indexes[tileID.overscaledZ].erase(tileID);
	indexes[tileID.overscaledZ].emplace(tileID, TileLayerIndex(tileID, bucket.symbolInstances, bucket.bucketInstanceId));

So the good news is nothing's broken and we just get a free, easy performance win! 😅

/cc @tobrun @ansis @friedbunny

[ChrisLoer]

Fixed in #11812.