WinRT: bug and data-integrity fixes for SDL_GetPrefPath()

This change does a few things, all with regards to the WinRT implementation of
SDL_GetPrefPath():

1. it fixes a bug whereby SDL_GetPrefPath() did not create the directory it
returned.  On other SDL platforms, SDL_GetPrefPath() will create separate
directories for its 'org' and 'app' folders.  Without this, attempts to create
files in the pref-path would fail, unless those directories were first created
by the app, or by some other library the app used.  This change makes sure
that these directories get created, before SDL_GetPrefPath() returns to its
caller(s).


2. it defaults to having SDL_GetPrefPath() return a WinRT 'Local' folder
on all platforms.  Previously, for Windows Store apps, it would have used a
different, 'Roaming' folder.  Files in Roaming folders can be automatically,
and synchronized across multiple devices by Windows.  This synchronization can
happen while the app runs, with new files being copied into a running app's
pref-path.  Unless an app is specifically designed to handle this scenario,
there is a chance that save-data could be overwritten in unwanted or
unexpected ways.

The default is now to use a Local folder, which does not get synchronized, and
which is arguably a bit safer to use.  Apps that wish to use Roaming folders
can do so by setting SDL_HINT_WINRT_PREF_PATH_ROOT to "roaming", however it
is recommended that one first read Microsoft's documentation for Roaming
files, a link to which is provided in README-winrt.md.

To preserve older pref-path selection behavior (found in SDL 2.0.3, as well as
many pre-2.0.4 versions of SDL from hg.libsdl.org), which uses a Roaming path
in Windows Store apps, and a Local path in Windows Phone, set
SDL_HINT_WINRT_PREF_PATH_ROOT to "old".

Please note that Roaming paths are not supported on Windows Phone 8.0, due to
limitations in the OS itself.  Attempts to use this will fail.
(Windows Phone 8.1 does not have this limitation, however.)


3. It makes SDL_GetPrefPath(), when on Windows Phone 8.0, and when
SDL_HINT_WINRT_PREF_PATH_ROOT is set to "roaming", return NULL, rather than
silently defaulting to a Local path (then switching to a Roaming path if and
when the user upgraded to Windows Phone 8.1).

include/SDL_hints.h

@@ -495,16 +495,25 @@ extern "C" {
  *  \brief  A variable that dictates what SDL_GetPrefPath() returns in WinRT apps.
  *
  *  The variable can be set to the following values:
- *    "local"   - Use the app's 'local' folder to store data; default for
- *                Windows Phone apps.
- *    "roaming" - Use the app's 'roaming' folder to store data; default for
- *                Windows Store (non-Phone) apps.  On Windows Phone 8.0, this
- *                setting will be ignored (and the 'local' folder will be used
- *                instead), as the OS does not support roaming folders.
+ *   * "local"   - Use the app's 'local' folder to store data.
+ *   * "roaming" - Use the app's 'roaming' folder to store data.
+ *                 On Windows Phone 8.0, this setting is not supported due to
+ *                 limitations in the OS itself.  Attempts to use this (via
+ *                 SDL_GetPrefPath()) on Windows Phone 8.0 will fail, with
+ *                 SDL_GetPrefPath() returning NULL.  (Windows Phone 8.1 does,
+ *                 however, support roaming folders.)
+ *   * "old"     - Use the app's 'local' folder on Windows Phone, and 'roaming'
+ *                 on non-Phone versions of WinRT.  This mimics behavior found
+ *                 in SDL 2.0.3's implementation of SDL_GetPrefPath() for WinRT
+ *                 (and was changed for SDL 2.0.4, further details of which are
+ *                 in the "Caveats" section of SDL's
+ *                 [WinRT README file](README-winrt.md).
  *
- *  Details on 'local' verses 'roaming' folders can be found on MSDN, in the
- *  documentation for WinRT's Windows.Storage.ApplicationData class, which is
- *  available at http://msdn.microsoft.com/en-us/library/windows/apps/windows.storage.applicationdata
+ *  The default is to use the app's "local" folder.
+ *
+ *  Details on 'local' verses 'roaming' folders can be found on MSDN, in
+ *  the documentation for WinRT's Windows.Storage.ApplicationData class,
+ *  (available at http://msdn.microsoft.com/en-us/library/windows/apps/windows.storage.applicationdata ).
  *
  *  The application's local and roaming paths may, alternatively, be retrieved
  *  via the SDL_WinRTGetFSPathUTF8() and SDL_WinRTGetFSPathUNICODE() functions,