I think someone confused these with the actual token and bounding box
registers in PE, which were added later. In CP they never did anything
and it's suspicious that they have the same addresses as their PE
counterparts. On real hardware they always read as zero.

Many games call GXSetGPFifo() without first waiting for the GP to finish
consuming outstanding commands in the previous GP fifo. Normally,
Dolphin runs OpcodeDecoding in 1000-cycle time slices. In that time
frame, GXSetGPFifo() has probably completed and the GP read pointer now
points to entirely new memory. If the last GP fifo copy ended in an
incomplete command, the new GP fifo would most likely desync for a
while. To avoid all this, give the GP a time slice right now to copy the
remaining data from the previous GP fifo.

Makes the use of the interface a little less noisy, especially given
how much of the interface depends on an instance being present.

Given how many member functions make use of the system instance,
it's likely just better to pass the system instance in on construction.

Makes the interface a little less noisy to use.

VideoCommon: Reword the unknown opcode error message

When faced with this error, users often don't try disabling dual core,
even though the error message suggests it. Perhaps the message is just
too long and lists too many things?

To try to improve the situation, I'm rewording the message and making it
say different things depending on what settings you are using.