Add SetTraceLogCallback to enable users setting custom logging (#597)

src/raylib.h

@@ -71,6 +71,8 @@
 #ifndef RAYLIB_H
 #define RAYLIB_H
 
+#include <stdarg.h>                             // Required for va_list
+
 #if defined(_WIN32) && defined(BUILD_LIBTYPE_SHARED)
     #define RLAPI __declspec(dllexport)         // We are building raylib as a Win32 shared library (.dll)
 #elif defined(_WIN32) && defined(USE_LIBTYPE_SHARED)
@@ -727,6 +729,9 @@ typedef enum {
     HMD_SONY_PSVR
 } VrDeviceType;
 
+// Callbacks to be implemented by users
+typedef void (*TraceLogCallback)(int msgType, const char *text, va_list args);
+
 #ifdef __cplusplus
 extern "C" {            // Prevents name mangling of functions
 #endif
@@ -796,6 +801,7 @@ RLAPI Color Fade(Color color, float alpha);                       // Color fade-
 RLAPI void ShowLogo(void);                                        // Activate raylib logo at startup (can be done with flags)
 RLAPI void SetConfigFlags(unsigned char flags);                   // Setup window configuration flags (view FLAGS)
 RLAPI void SetTraceLog(unsigned char types);                      // Enable trace log message types (bit flags based)
+RLAPI void SetTraceLogCallback(TraceLogCallback callback);        // Set a trace log callback to enable custom logging bypassing raylib's one
 RLAPI void TraceLog(int logType, const char *text, ...);          // Show trace log messages (LOG_INFO, LOG_WARNING, LOG_ERROR, LOG_DEBUG)
 RLAPI void TakeScreenshot(const char *fileName);                  // Takes a screenshot of current screen (saved a .png)
 RLAPI int GetRandomValue(int min, int max);                       // Returns a random value between min and max (both included)