Fixed compiling of three test programs with C++.

test/testgles.c

@@ -173,7 +173,7 @@ main(int argc, char *argv[])
         quit(2);
     }
 
-    context = SDL_calloc(state->num_windows, sizeof(context));
+    context = (SDL_GLContext *)SDL_calloc(state->num_windows, sizeof(context));
     if (context == NULL) {
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Out of memory!\n");
         quit(2);