Support custom memory management macros

Users can define their custom memory management macros.

NOTE: Most external libraries support custom macros in the same way, raylib should redefine those macros to raylib ones, to unify custom memory loading. That redefinition is only implemented as example for stb_image.h in [textures] module.

src/raylib.h

@@ -99,6 +99,17 @@
 // Shader and material limits
 #define MAX_SHADER_LOCATIONS    32      // Maximum number of predefined locations stored in shader struct
 #define MAX_MATERIAL_MAPS       12      // Maximum number of texture maps stored in shader struct
+
+// Allow custom memory allocators
+#ifndef RL_MALLOC
+    #define RL_MALLOC(sz)       malloc(sz)
+#endif
+#ifndef RL_CALLOC
+    #define RL_CALLOC(n,sz)     calloc(n,sz)
+#endif
+#ifndef RL_FREE
+    #define RL_FREE(p)          free(p)
+#endif
 
 // NOTE: MSC C++ compiler does not support compound literals (C99 feature)
 // Plain structures in C++ (without constructors) can be initialized from { } initializers.