WinRT: added SDL_HINT_WINRT_PREF_PATH_ROOT

SDL_HINT_WINRT_PREF_PATH_ROOT allows WinRT apps to alter the path that
SDL_GetPrefPath() returns.  Setting it to "local" uses the app's
OS-defined Local folder, setting it to "roaming" uses the app's OS-defined
Roaming folder.

Roaming folder support is not available in Windows Phone 8.0.  Attempts to
make SDL_GetPrefPath() return a Roaming folder on this OS will be ignored.

Various bits of documentation on this were added to SDL_hints.h, and to
README-winrt.md

docs/README-winrt.md

@@ -116,6 +116,32 @@ Here is a rough list of what works, and what doens't:
 
 
 
+Caveats
+-------
+
+#### SDL_GetPrefPath() usage
+
+SDL_GetPrefPath() is available for use in WinRT apps, however the following
+should be noted:
+
+1. It will return different path types, by default, depending on the WinRT
+   platform.  Windows Phone apps will default to using the app's "local" path,
+   whereas Windows Store (i.e. non-Phone) apps will default to using the app's
+   "roaming" path.  This behavior can be changed by calling SDL_SetHint() with
+   the key, SDL_HINT_WINRT_PREF_PATH_ROOT, and a value of either "local" or
+   "roaming".
+
+2. Windows Phone 8.0 does not provide apps access to a "roaming" folder.
+   Attempts to make SDL_GetPrefPath() return a roaming folder on Windows
+   Phone 8.0 will be ignored (and a path inside the "local" folder will be
+   used instead).
+
+Further details on this can be found in the documentation for
+SDL_HINT_WINRT_PREF_PATH_ROOT, in SDL_hints.h, as well as the docs for
+SDL_WinRT_Path, SDL_WinRTGetFSPathUNICODE, and SDL_WinRTGetFSPathUTF8,
+in SDL_system.h.
+
+
 
 Setup, High-Level Steps
 -----------------------

include/SDL_hints.h

@@ -491,6 +491,27 @@ extern "C" {
  */
 #define SDL_HINT_WINRT_HANDLE_BACK_BUTTON "SDL_WINRT_HANDLE_BACK_BUTTON"
 
+/**
+ *  \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.
+ *
+ *  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 application's local and roaming paths may, alternatively, be retrieved
+ *  via the SDL_WinRTGetFSPathUTF8() and SDL_WinRTGetFSPathUNICODE() functions,
+ *  which are defined in SDL_system.h.
+ */
+#define SDL_HINT_WINRT_PREF_PATH_ROOT "SDL_WINRT_PREF_PATH_ROOT"
+
 /**
  *  \brief  A variable that dictates policy for fullscreen Spaces on Mac OS X.
  *

src/filesystem/winrt/SDL_sysfilesystem.cpp

@@ -28,6 +28,7 @@
 extern "C" {
 #include "SDL_filesystem.h"
 #include "SDL_error.h"
+#include "SDL_hints.h"
 #include "SDL_stdinc.h"
 #include "SDL_system.h"
 #include "../../core/windows/SDL_windows.h"
@@ -149,13 +150,26 @@ SDL_GetPrefPath(const char *org, const char *app)
      * compatibility with Windows Phone 8.0, and with app-installs that have
      * been updated from 8.0-based, to 8.1-based apps.
      */
-    const char * srcPath = SDL_WinRTGetFSPathUTF8(SDL_WINRT_PATH_LOCAL_FOLDER);
+    SDL_WinRT_Path pathType = SDL_WINRT_PATH_LOCAL_FOLDER;
 #else
     /* A 'Roaming' folder is available on Windows 8 and 8.1.  Use that.
      */
-    const char * srcPath = SDL_WinRTGetFSPathUTF8(SDL_WINRT_PATH_ROAMING_FOLDER);
+    SDL_WinRT_Path pathType = SDL_WINRT_PATH_ROAMING_FOLDER;
 #endif
 
+    const char * hint = SDL_GetHint(SDL_HINT_WINRT_PREF_PATH_ROOT);
+    if (hint) {
+        if (SDL_strcasecmp(hint, "local") == 0) {
+            pathType = SDL_WINRT_PATH_LOCAL_FOLDER;
+        }
+#if (WINAPI_FAMILY != WINAPI_FAMILY_PHONE_APP) || (NTDDI_VERSION > NTDDI_WIN8)
+        else if (SDL_strcasecmp(hint, "roaming") == 0) {
+            pathType = SDL_WINRT_PATH_ROAMING_FOLDER;
+        }
+#endif
+    }
+
+    const char * srcPath = SDL_WinRTGetFSPathUTF8(pathType);
     size_t destPathLen;
     char * destPath = NULL;