Enabled SDL_image's support for PNG and TIFF files and improved the

AssetManager with a hashing function, a linked list structure to store
the hashes as well as a reference counter to support multiple users
sharing the same asset.

include/constants.h

@@ -1,6 +1,10 @@
 #ifndef CONSTANTS_H
 #define CONSTANTS_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif // !__cplusplus
+
 #define INTERNAL static
 
 #define MAX_OBJ_COUNT 10000
@@ -8,4 +12,8 @@
 #define HASH_PRIME_01 1987
 #define HASH_PRIME_02 7919
 
+#ifdef __cplusplus
+}
+#endif // !__cplusplus
+
 #endif // !CONSTANTS_H