Fix threading issue with vertex loader JIT.

VertexLoader::VertexLoader was setting loop_counter, a *static*
variable, to 0.  This was nonsensical, but harmless until I started to
run it on a separate thread, where it had a chance of interfering with a
running vertex translator.

Switch to just using a register for the loop counter.
Switch to an unordered_map as a micro-optimization.
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.  (Could calculate it from preprocess rather than taking as an
argument, but that's uglier.)  I also get rid of an unused overload.
Rejigger some FIFO buffer variables to be more rational.

videoBuffer -> s_video_buffer
size -> s_video_buffer_write_ptr
g_pVideoData -> g_video_buffer_read_ptr (impl moved to Fifo.cpp)

This eradicates the wonderful use of 'size' as a global name, and makes
it clear that s_video_buffer_write_ptr and g_video_buffer_read_ptr are
the two ends of the FIFO buffer s_video_buffer.

Oh, and remove a useless namespace {}.
Make ReadDataFromFifo static.
Derp