filesystem: SDL_GetBasePath() now follows the SDL_GetStringRule.

It also now caches at the higher level, so the platform-specific bits don't
change their interface much.

A little code hygiene work was applied to some of the platform bits on top of
this.

Reference Issue #10229.

test/testfilesystem.c

@@ -62,7 +62,7 @@ int main(int argc, char *argv[])
 {
     SDLTest_CommonState *state;
     char *pref_path;
-    char *base_path;
+    const char *base_path;
 
     /* Initialize test framework */
     state = SDLTest_CommonCreateState(argv, 0);
@@ -145,8 +145,6 @@ int main(int argc, char *argv[])
         }
     }
 
-    SDL_free(base_path);
-
     SDL_Quit();
     SDLTest_CommonDestroyState(state);
     return 0;