Update source to SDL2 2.0.12

Edward Rudd

2020-05-02 03f8528315fa46c95991a34f3325d7b33ae5538c

 source/VisualC-WinRT/UWP_VS2015/SDL-UWP.vcxproj.filters

@@ -626,6 +626,9 @@
    <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\generic\SDL_syssem.c">

      <Filter>Source Files</Filter>

    </ClCompile>