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.

src/filesystem/gdk/SDL_sysfilesystem.cpp

@@ -31,7 +31,7 @@
 #include <XGameSaveFiles.h>
 
 char *
-SDL_GetBasePath(void)
+SDL_SYS_GetBasePath(void)
 {
     /* NOTE: This function is a UTF8 version of the Win32 SDL_GetBasePath()!
      * The GDK actually _recommends_ the 'A' functions over the 'W' functions :o