Integrated sdl2-cs bindings for sdl2

This is using the sdl2/pure branch of https://github.com/thefiddler/SDL2-CS.git which contains the SDL2 bindings without MiniTK.

Source/OpenTK/OpenTK.csproj

@@ -782,6 +782,10 @@
     <Compile Include="IntPtrEqualityComparer.cs" />
     <Compile Include="Input\GamePadButton.cs" />
     <Compile Include="Input\GamePadAxis.cs" />
+    <None Include="Platform\SDL2\sdl2-cs\LICENSE" />
+    <None Include="Platform\SDL2\sdl2-cs\README" />
+    <Compile Include="Platform\SDL2\sdl2-cs\src\LPUtf8StrMarshaler.cs" />
+    <Compile Include="Platform\SDL2\sdl2-cs\src\SDL2.cs" />
   </ItemGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <PropertyGroup>
@@ -790,4 +794,7 @@
     <PostBuildEvent>
     </PostBuildEvent>
   </PropertyGroup>
+  <ItemGroup>
+    <Folder Include="Platform\SDL2\" />
+  </ItemGroup>
 </Project>

Source/OpenTK/Platform/SDL2/sdl2-cs/src/LPUtf8StrMarshaler.cs

@@ -28,7 +28,7 @@ using System;
 using System.Text;
 using System.Runtime.InteropServices;
 
-namespace SDL2
+namespace OpenTK.Platform.SDL2
 {
 	internal unsafe class LPUtf8StrMarshaler : ICustomMarshaler
 	{

Source/OpenTK/Platform/SDL2/sdl2-cs/src/SDL2.cs

@@ -31,7 +31,7 @@ using System;
 using System.Runtime.InteropServices;
 #endregion
 
-namespace SDL2
+namespace OpenTK.Platform.SDL2
 {
 	/// <summary>
 	/// Entry point for all SDL-related (non-extension) types and methods