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

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.

src/mbgl/renderer/render_layer.cpp

@@ -71,10 +71,13 @@ RenderLayer::RenderTiles RenderLayer::filterRenderTiles(RenderTiles tiles, Filte
             continue;
         }
 
-        if (tile.tile.getBucket(*baseImpl)) {
+        if (Bucket* bucket = tile.tile.getBucket(*baseImpl)) {
             tile.used = true;
             tile.needsClipping |= needsClipping_;
             filtered.emplace_back(tile);
+            if (tile.tile.isComplete()) {
+                updateBucketPaintProperties(bucket);
+            }
         }
     }
     return filtered;
@@ -84,6 +87,10 @@ void RenderLayer::markContextDestroyed() {
     // no-op
 }
 
+void RenderLayer::updateBucketPaintProperties(Bucket*) const {
+    // no-op
+}
+
 void RenderLayer::checkRenderability(const PaintParameters& parameters,
                                      const uint32_t activeBindingCount) {
     // Only warn once for every layer.