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;

test/testutils.c

@@ -23,11 +23,9 @@
 char *
 GetNearbyFilename(const char *file)
 {
-    char *base;
+    const char *base = SDL_GetBasePath();
     char *path;
 
-    base = SDL_GetBasePath();
-
     if (base) {
         SDL_IOStream *rw;
         size_t len = SDL_strlen(base) + SDL_strlen(file) + 1;
@@ -35,12 +33,10 @@ GetNearbyFilename(const char *file)
         path = SDL_malloc(len);
 
         if (!path) {
-            SDL_free(base);
             return NULL;
         }
 
         (void)SDL_snprintf(path, len, "%s%s", base, file);
-        SDL_free(base);
 
         rw = SDL_IOFromFile(path, "rb");
         if (rw) {