Changes to allow LoadCPReg to work in a preprocess mode which affects a separate state.

This state will be used to calculate sizes for skipping over commands on
a separate thread.  An alternative to having these state variables would
be to have the preprocessor stash "state as we go" somewhere, but I
think that would be much uglier.

GetVertexSize now takes an extra argument to determine which state to
use, as does FifoCommandRunnable, which calls it.  While I'm modifying
FifoCommandRunnable, I also change it to take a buffer and size as
parameters rather than using g_pVideoData, which will also be necessary
later.  I also get rid of an unused overload.

Source/Core/VideoCommon/VertexLoaderManager.h

@@ -16,7 +16,7 @@ namespace VertexLoaderManager
 
 	void MarkAllDirty();
 
-	int GetVertexSize(int vtx_attr_group);
+	int GetVertexSize(int vtx_attr_group, bool preprocess);
 	// Returns false if buf_size is insufficient.
 	bool RunVertices(int vtx_attr_group, int primitive, int count, size_t buf_size, bool skip_drawing = false);