Update source to SDL2 2.0.12

Edward Rudd

2020-05-02 03f8528315fa46c95991a34f3325d7b33ae5538c

 source/VisualC-WinRT/WinPhone81_VS2013/SDL-WinPhone81.vcxproj.filters

@@ -593,6 +593,9 @@
    <ClCompile Include="..\..\src\stdlib\SDL_string.c">

      <Filter>Source Files</Filter>

    </ClCompile>

    <ClCompile Include="..\..\src\stdlib\SDL_strtokr.c">

      <Filter>Source Files</Filter>

    </ClCompile>

    <ClCompile Include="..\..\src\thread\SDL_thread.c">

      <Filter>Source Files</Filter>

    </ClCompile>