[core] shader program must always match bucket in render symbol layer

[pozdnyakov]

Before this change, RenderSymbolLayer with updated style was trying to render
symbols using the previous bucket (with paint property binders that matched a previous program).

Now, symbol bucket caches the latest corresponding paint properties (caching is happening on
complete tiles only). As a result, RenderSymbolLayer always picks the shader program and
its parameters in sync with the obtained bucket.

Fixes: #13407

[ChrisLoer]

The problem with storing the paint property values in the bucket is that if you modify just a paint property but no layout properties, the bucket doesn't get re-generated -- so with this code, your paint property change wouldn't show up until something else triggered a tile reload. This is why I tried to save just the constant Bitset (for shader selection) instead of the values themselves in the bucket-bitset branch -- although it's somewhat more complicated.

On top of the "paint changes, layout doesn't" case, there's also the case where someone changes both paint and layout properties at the same time, triggering re-layout. The current (and intended) behavior is that the paint property change takes effect immediately, while the layout change doesn't take effect until the tile reloads. You could definitely argue that it would be more consistent for both changes to take place at the same time, but I think so far we've prioritized making constant-value paint property changes instantaneous.

Also, however we fix this, we should do it across all the layer types, right?

[pozdnyakov]

@ChrisLoer Thanks for your comments! Paint properties are now updated for the complete tile buckets. PTAL.

[pozdnyakov]

Also, however we fix this, we should do it across all the layer types, right?

Sure thing! Once we agree on the approach I'd be happy to spread it to other layer types.

[tobrun]

#13705 adds a manual regression test.
Validated that this PR fixes the issue from #13407

[ChrisLoer]

Awesome!