Added thread-affinity checks to SDL2 renderer.

If a call is made into a graphics resource that has thread-affinity, from a thread other than the one that created the graphics device, an exception will now be thrown to make debugging easier.

OpenRA.Platforms.Default/OpenRA.Platforms.Default.csproj

@@ -50,6 +50,7 @@
     <Compile Include="FrameBuffer.cs" />
     <Compile Include="MultiTapDetection.cs" />
     <Compile Include="Texture.cs" />
+    <Compile Include="ThreadAffine.cs" />
     <Compile Include="VertexBuffer.cs" />
     <Compile Include="OpenAlSoundEngine.cs" />
   </ItemGroup>