[core] Clear tile pyramid for custom source only if there is a significant change

Clear tile pyramid only if updated source has different tile options,
zoom range or initialization state for a custom tile loader.

src/mbgl/renderer/sources/render_custom_geometry_source.cpp

@@ -23,7 +23,14 @@ void RenderCustomGeometrySource::update(Immutable<style::Source::Impl> baseImpl_
                                  const TileParameters& parameters) {
     if (baseImpl != baseImpl_) {
         std::swap(baseImpl, baseImpl_);
-        tilePyramid.clearAll();
+
+        // Clear tile pyramid only if updated source has different tile options,
+        // zoom range or initialization state for a custom tile loader.
+        auto newImpl = staticImmutableCast<style::CustomGeometrySource::Impl>(baseImpl);
+        auto currentImpl = staticImmutableCast<style::CustomGeometrySource::Impl>(baseImpl_);
+        if (*newImpl != *currentImpl) {
+            tilePyramid.clearAll();
+        }
     }
 
     enabled = needsRendering;

src/mbgl/style/sources/custom_geometry_source.cpp

@@ -14,11 +14,10 @@
 namespace mbgl {
 namespace style {
 
-CustomGeometrySource::CustomGeometrySource(std::string id,
-                                       const CustomGeometrySource::Options options)
+CustomGeometrySource::CustomGeometrySource(std::string id, CustomGeometrySource::Options options)
     : Source(makeMutable<CustomGeometrySource::Impl>(std::move(id), options)),
-    loader(std::make_unique<Actor<CustomTileLoader>>(Scheduler::GetBackground(), options.fetchTileFunction, options.cancelTileFunction)) {
-}
+      loader(std::make_unique<Actor<CustomTileLoader>>(
+          Scheduler::GetBackground(), options.fetchTileFunction, options.cancelTileFunction)) {}
 
 CustomGeometrySource::~CustomGeometrySource() = default;
 

src/mbgl/style/sources/custom_geometry_source_impl.cpp

@@ -13,6 +13,10 @@ CustomGeometrySource::Impl::Impl(std::string id_, CustomGeometrySource::Options
 CustomGeometrySource::Impl::Impl(const Impl& impl, ActorRef<CustomTileLoader> loaderRef_)
     : Source::Impl(impl), tileOptions(impl.tileOptions), zoomRange(impl.zoomRange), loaderRef(loaderRef_) {}
 
+bool CustomGeometrySource::Impl::operator!=(const Impl& other) const noexcept {
+    return tileOptions != other.tileOptions || zoomRange != other.zoomRange || bool(loaderRef) != bool(other.loaderRef);
+}
+
 optional<std::string> CustomGeometrySource::Impl::getAttribution() const {
     return {};
 }

src/mbgl/style/sources/custom_geometry_source_impl.hpp

@@ -18,6 +18,7 @@ public:
     Immutable<CustomGeometrySource::TileOptions> getTileOptions() const;
     Range<uint8_t> getZoomRange() const;
     optional<ActorRef<CustomTileLoader>> getTileLoader() const;
+    bool operator!=(const Impl&) const noexcept;
 
 private:
     Immutable<CustomGeometrySource::TileOptions> tileOptions;