Const corectness for LoadShaderCode (#977)

* Const corectness for LoadShaderCode

* Added const for standalone variant as well.

src/raylib.h

@@ -1304,7 +1304,7 @@ RLAPI RayHitInfo GetCollisionRayGround(Ray ray, float groundHeight);
 // Shader loading/unloading functions
 RLAPI char *LoadText(const char *fileName);                               // Load chars array from text file
 RLAPI Shader LoadShader(const char *vsFileName, const char *fsFileName);  // Load shader from files and bind default locations
-RLAPI Shader LoadShaderCode(char *vsCode, char *fsCode);                  // Load shader from code strings and bind default locations
+RLAPI Shader LoadShaderCode(const char *vsCode, const char *fsCode);                  // Load shader from code strings and bind default locations
 RLAPI void UnloadShader(Shader shader);                                   // Unload shader from GPU memory (VRAM)
 
 RLAPI Shader GetShaderDefault(void);                                      // Get default shader

src/rlgl.h

@@ -519,7 +519,7 @@ RLAPI void rlUnloadMesh(Mesh mesh);                                       // Unl
 // Shader loading/unloading functions
 RLAPI char *LoadText(const char *fileName);                               // Load chars array from text file
 RLAPI Shader LoadShader(const char *vsFileName, const char *fsFileName);  // Load shader from files and bind default locations
-RLAPI Shader LoadShaderCode(char *vsCode, char *fsCode);                  // Load shader from code strings and bind default locations
+RLAPI Shader LoadShaderCode(const char *vsCode, const char *fsCode);                  // Load shader from code strings and bind default locations
 RLAPI void UnloadShader(Shader shader);                                   // Unload shader from GPU memory (VRAM)
 
 RLAPI Shader GetShaderDefault(void);                                      // Get default shader
@@ -2986,7 +2986,7 @@ Shader LoadShader(const char *vsFileName, const char *fsFileName)
 
 // Load shader from code strings
 // NOTE: If shader string is NULL, using default vertex/fragment shaders
-Shader LoadShaderCode(char *vsCode, char *fsCode)
+Shader LoadShaderCode(const char *vsCode, const char *fsCode)
 {
     Shader shader = { 0 };
     shader.locs = (int *)RL_CALLOC(MAX_SHADER_LOCATIONS, sizeof(int));