ThreadedGraphicsContext improvements.

- VertexBuffer interface redefined to remove an IntPtr overload for SetData. This removes some unsafe code in TerrainSpriteLayer. This also allows the ThreadedVertexBuffer to use a buffer and post these calls, meaning the SetData call can now be non-blocking.
- ThreadedTexture SetData now checks the incoming array size. As the arrays sent here are usually large (megabytes) this allows us to avoid creating temp arrays in the LOH and skip Array.Copy calls on large arrays. This means the call is now blocking more often, but significantly reduces memory churn and GC Gen2 collections.

OpenRA.Game/Graphics/PlatformInterfaces.cs

@@ -103,8 +103,7 @@ namespace OpenRA
 	{
 		void Bind();
 		void SetData(T[] vertices, int length);
-		void SetData(T[] vertices, int start, int length);
-		void SetData(IntPtr data, int start, int length);
+		void SetData(T[] vertices, int offset, int start, int length);
 	}
 
 	public interface IShader