WinRT: added Win10/UWP (Universal Windows Platform) support

"UWP" appears to be Microsoft's new name for WinRT/Windows-Store APIs.

This set of changes updates SDL's WinRT backends to support the Win10 flavor
of WinRT.  It has been tested on Win10 on a desktop.  In theory, it should
also support Win10 on other devices (phone, Xbox One, etc.), however further
patches may be necessary.

This adds:
- a set of MSVC 2015 project files, for use in creating UWP apps
- modifications to various pieces of SDL, in order to compile via MSVC 2015 +
  the Win10 API set
- enables SDL_Window resizing and programmatic-fullscreen toggling, when using
  the WinRT backend
- WinRT README updates

src/video/winrt/SDL_winrtvideo_cpp.h

@@ -75,9 +75,9 @@ extern "C" Uint32 WINRT_DetectWindowFlags(SDL_Window * window);  /* detects flag
 #define WINRT_DISPLAY_PROPERTY(NAME) (Windows::Graphics::Display::DisplayProperties::NAME)
 #endif
 
-/* Converts DIPS to physical pixels */
-#define WINRT_DIPS_TO_PHYSICAL_PIXELS(DIPS) ((int)(0.5f + (((float)(DIPS) * (float)WINRT_DISPLAY_PROPERTY(LogicalDpi)) / 96.f)))
-
+/* Converts DIPS to/from physical pixels */
+#define WINRT_DIPS_TO_PHYSICAL_PIXELS(DIPS)     ((int)(0.5f + (((float)(DIPS) * (float)WINRT_DISPLAY_PROPERTY(LogicalDpi)) / 96.f)))
+#define WINRT_PHYSICAL_PIXELS_TO_DIPS(PHYSPIX)  (((float)(PHYSPIX) * 96.f)/WINRT_DISPLAY_PROPERTY(LogicalDpi))
 
 /* Internal window data */
 struct SDL_WindowData