Add SetRandomSeed(unsigned int seed) function (#1994)

Specifying a fixed seed for the random number generator is often
used in games for various reasons.

By adding an api function for seeding the random number generator
we solve two different problems regarding the seeding:

1) The underlying RNG implementation does not leak to client code
   (as would be the case if we called srand directly from the
   client code)
2) Seeding the RNG would be simple from other programming languages
   (especially in cases where calling libc functions is non-trivial)

src/core.c

@@ -2669,6 +2669,12 @@ int GetRandomValue(int min, int max)
     return (rand()%(abs(max - min) + 1) + min);
 }
 
+// Set the seed for the random number generator
+void SetRandomSeed(unsigned int seed)
+{
+    srand(seed);
+}
+
 // Check if the file exists
 bool FileExists(const char *fileName)
 {

src/raylib.h

@@ -1009,6 +1009,7 @@ RLAPI double GetTime(void);                                       // Get elapsed
 
 // Misc. functions
 RLAPI int GetRandomValue(int min, int max);                       // Get a random value between min and max (both included)
+RLAPI void SetRandomSeed(unsigned int seed);                      // Set the seed for the random number generator
 RLAPI void TakeScreenshot(const char *fileName);                  // Takes a screenshot of current screen (filename extension defines format)
 RLAPI void SetConfigFlags(unsigned int flags);                    // Setup init configuration flags (view FLAGS)