Make C++ mutex methods constant.

Closes #1729.

Co-authored-by: Diego Fonseca <fonseca.diego.feeshy@gmail.com>

project/include/system/Mutex.h

@@ -13,9 +13,9 @@ namespace lime {
 			Mutex ();
 			~Mutex ();
 
-			bool Lock ();
-			bool TryLock ();
-			bool Unlock ();
+			bool Lock () const;
+			bool TryLock () const;
+			bool Unlock () const;
 
 		private:
 

project/src/backend/sdl/SDLMutex.cpp

@@ -23,7 +23,7 @@ namespace lime {
 	}
 
 
-	bool Mutex::Lock () {
+	bool Mutex::Lock () const {
 
 		if (mutex) {
 
@@ -36,7 +36,7 @@ namespace lime {
 	}
 
 
-	bool Mutex::TryLock () {
+	bool Mutex::TryLock () const {
 
 		if (mutex) {
 
@@ -49,7 +49,7 @@ namespace lime {
 	}
 
 
-	bool Mutex::Unlock () {
+	bool Mutex::Unlock () const {
 
 		if (mutex) {