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/render/direct3d11/SDL_render_d3d11.c

@@ -2367,7 +2367,7 @@ D3D11_UpdateVertexBuffer(SDL_Renderer *renderer,
     } else {
         SAFE_RELEASE(rendererData->vertexBuffer);
 
-        vertexBufferDesc.ByteWidth = dataSizeInBytes;
+        vertexBufferDesc.ByteWidth = (UINT) dataSizeInBytes;
         vertexBufferDesc.Usage = D3D11_USAGE_DYNAMIC;
         vertexBufferDesc.BindFlags = D3D11_BIND_VERTEX_BUFFER;
         vertexBufferDesc.CPUAccessFlags = D3D11_CPU_ACCESS_WRITE;