[core] Evict unused font stacks from GlyphManager

include/mbgl/util/font_stack.hpp

@@ -1,7 +1,11 @@
 #pragma once
 
+#include <mbgl/util/immutable.hpp>
+#include <mbgl/style/layer.hpp>
+
 #include <string>
 #include <vector>
+#include <set>
 
 namespace mbgl {
 
@@ -14,4 +18,7 @@ struct FontStackHash {
     std::size_t operator()(const FontStack&) const;
 };
 
+// Statically evaluate layer properties to determine what font stacks are used.
+std::set<FontStack> fontStacks(const std::vector<Immutable<style::Layer::Impl>>&);
+
 } // namespace mbgl