[core] Initial state of Source::Impl::enabled must be true

Until Style::recalculate() is called to check that there are no visible layers using the source, we have to assume there are. Otherwise, Style::isLoaded() can return a false positive.

src/mbgl/style/source_impl.hpp

@@ -74,7 +74,11 @@ public:
     const std::string id;
 
     bool loaded = false;
-    bool enabled = false;
+
+    // Tracks whether the source is used by any layers visible at the current zoom level. Must
+    // be initialized to true so that Style::isLoaded() does not produce false positives if
+    // called before Style::recalculate().
+    bool enabled = true;
 
 protected:
     void invalidateTiles();

src/mbgl/style/style.cpp

@@ -305,7 +305,9 @@ bool Style::isLoaded() const {
     }
 
     for (const auto& source: sources) {
-        if (source->baseImpl->enabled && !source->baseImpl->isLoaded()) return false;
+        if (source->baseImpl->enabled && !source->baseImpl->isLoaded()) {
+            return false;
+        }
     }
 
     if (!spriteStore->isLoaded()) {

test/style/style.cpp

@@ -17,6 +17,10 @@ TEST(Style, UnusedSource) {
     auto now = Clock::now();
 
     style.setJSON(util::read_file("test/fixtures/resources/style-unused-sources.json"));
+
+    // If we haven't calculated whether the source is used, we have to assume it is used.
+    EXPECT_FALSE(style.isLoaded());
+
     style.cascade(now, MapMode::Still);
     style.recalculate(0, now, MapMode::Still);