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/VideoState.cpp

@@ -68,5 +68,6 @@ void VideoCommon_RunLoop(bool enable)
 void VideoCommon_Init()
 {
 	memset(&g_main_cp_state, 0, sizeof(g_main_cp_state));
+	memset(&g_preprocess_cp_state, 0, sizeof(g_preprocess_cp_state));
 	memset(texMem, 0, TMEM_SIZE);
 }