[core] Fix misc-unconventional-assign-operator errors in header files

As reported by clang-tidy-8.

src/mbgl/gl/state.hpp

@@ -20,11 +20,13 @@ public:
     State(Args&&... args) : params(std::forward_as_tuple(::std::forward<Args>(args)...)) {
     }
 
-    void operator=(const typename T::Type& value) {
+    State& operator=(const typename T::Type& value) {
         if (*this != value) {
             setCurrentValue(value);
             set(std::index_sequence_for<Args...>{});
         }
+
+        return *this;
     }
 
     bool operator==(const typename T::Type& value) const {

src/mbgl/gl/uniform.hpp

@@ -39,11 +39,13 @@ class UniformState {
 public:
     UniformState(UniformLocation location_ = -1) : location(location_) {}
 
-    void operator=(const Value& value) {
+    UniformState& operator=(const Value& value) {
         if (location >= 0 && (!current || *current != value)) {
             current = value;
             bindUniform(location, value);
         }
+
+        return *this;
     }
 
     UniformLocation location;