Fix for PSM SpriteEffect MGFX crash

On PSM, the default SpriteEffect bytecode causes a crash on launch due
to the shader loading pipeline not being able to correctly read the
stream's header info.

This fix is derived from goblinincident's fix listed here:
http://community.monogame.net/t/spritebatch-cs-effect-cs-this-does-not-appear-to-be-a-monogame-mgfx-file/1070