Code refactor: CombinerProgram()::getKey() returns reference instead of object

to avoid copy of the key to temp object when combiner added to combiners map.

src/CombinerKey.cpp

@@ -64,6 +64,13 @@ bool CombinerKey::isRectKey() const
 	return ((m_key.muxs0 >> 24) & 1) != 0;
 }
 
-void CombinerKey::read(std::istream & _is) {
+void CombinerKey::read(std::istream & _is)
+{
 	_is.read((char*)&m_key.mux, sizeof(m_key.mux));
 }
+
+const CombinerKey & CombinerKey::getEmpty()
+{
+	static CombinerKey emptyKey;
+	return emptyKey;
+}

src/CombinerKey.h

@@ -26,6 +26,8 @@ public:
 
 	void read(std::istream & _is);
 
+	static const CombinerKey & getEmpty();
+
 private:
 	gDPCombine m_key;
 };

src/Graphics/CombinerProgram.h

@@ -12,7 +12,7 @@ namespace graphics {
 		virtual void activate() = 0;
 		virtual void update(bool _force) = 0;
 
-		virtual CombinerKey getKey() const = 0;
+		virtual const CombinerKey & getKey() const = 0;
 
 		virtual bool usesTexture() const = 0;
 		virtual bool usesTile(u32 _t) const = 0;

src/Graphics/OpenGLContext/GLSL/glsl_CombinerProgramImpl.cpp

@@ -44,7 +44,7 @@ void CombinerProgramImpl::update(bool _force)
 		(*it)->update(_force);
 }
 
-CombinerKey CombinerProgramImpl::getKey() const
+const CombinerKey & CombinerProgramImpl::getKey() const
 {
 	return m_key;
 }

src/Graphics/OpenGLContext/GLSL/glsl_CombinerProgramImpl.h

@@ -31,7 +31,7 @@ namespace glsl {
 
 		void activate() override;
 		void update(bool _force) override;
-		CombinerKey getKey() const override;
+		const CombinerKey & getKey() const override;
 
 		bool usesTexture() const override;
 		bool usesTile(u32 _t) const override;

src/Graphics/ShaderProgram.h

@@ -10,7 +10,7 @@ namespace graphics {
 		virtual ~ShaderProgram() {}
 
 		void update(bool _force) override {}
-		CombinerKey getKey() const override {return CombinerKey();}
+		const CombinerKey & getKey() const override { return CombinerKey::getEmpty(); }
 		bool usesTexture() const override {return true;}
 		virtual bool usesTile(u32 _t) const override {return _t == 0 ? true : false;}
 		virtual bool usesShade() const override {return false;}