Added base-level project files for the Windows 8.0/RT variant of WinRT

Support for .png and .bmp files were added via this change.  PNG support was
added via the copies of libpng and zlib in the "external" directory.  No changes
to either external library (libpng or zlib) were required; the only changes
requred were the addition of Visual C++ project files.

Still TODO:
- add project files for Windows Phone 8
- add project files for Windows 8.1 (via VS2013-based project files)
- add support for other image formats (JPEG, WebP, etc.)

To enable:
1. add each 'VS2012' project file in VisualC-WinRT to the app's VS2012 solution
2. link the app to SDL_image via a Visual C++ Reference (right click on the
app's project, choose References, click Add New Reference, check the SDL_image
box, click OK).
Added base-level project files for the Windows Phone 8.0 variant of WinRT

Support for .png and .bmp files were added via this change. PNG support was
added via the copies of libpng and zlib in the "external" directory. No changes
to either external library (libpng or zlib) were required; the only changes
requred were the addition of Visual C++ project files.

Still TODO (for SDL_image on Windows Phone 8):
- add support for other image formats (JPEG, WebP, etc.)
- add support for Windows Phone 8.1, if needed (once enough details on it
emerge)

To enable:
1. add each 'VS2012' project file in VisualC-WinPhone to the app's VS2012
solution.
2. link the app to SDL_image via a Visual C++ Reference (right click on the
app's project, choose References, click Add New Reference, check the SDL_image
box, click OK).