Fixed compiling of three test programs with C++.

test/testgles2.c

@@ -546,7 +546,7 @@ main(int argc, char *argv[])
         return 0;
     }
 
-    context = SDL_calloc(state->num_windows, sizeof(context));
+    context = (SDL_GLContext *)SDL_calloc(state->num_windows, sizeof(context));
     if (context == NULL) {
         SDL_Log("Out of memory!\n");
         quit(2);
@@ -640,7 +640,7 @@ main(int argc, char *argv[])
         }
     }
 
-    datas = SDL_calloc(state->num_windows, sizeof(shader_data));
+    datas = (shader_data *)SDL_calloc(state->num_windows, sizeof(shader_data));
 
     /* Set rendering settings for each context */
     for (i = 0; i < state->num_windows; ++i) {