Remove 2d render settings cache completely, as it didn't seem to speed up things, but only introduced complexity and failures. Speed up text rendering under dx again, just as before in OpenGL by skipping too many render state resets. Unified all three setRenderStates2DMode methods.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3734 dfc29bdd-3216-0410-991c-e03cc46cb475