Fix for SetRenderTargets MRT usage scenario

When calling SetRenderTargets with MRT and then calling it again with
fewer targets than the MRT, the additional RT's in the MRT are not
correctly rebound in future SetRenderTargets calls.

This line fixes that.
Formatting fix
Merge pull request #203 from jasedeacon/monogame-sdl2

Fix for SetRenderTargets MRT usage scenario