Eat cycles during block transfers.

Tests show this takes a number very close to 1.9 times the number of bytes
in cycles @222.

Using that value caused a tiny fps drop in one area of the God of War
demo, which may be correct but might also cause complaints.  A lot of our
other timing is not accurate, so using a lower value (1.6) to be on the
safer side.

This may cause things to reschedule more accurately when games drawsync,
and improves the gpu/commands/blocktransfer test.
Avoid some magic numbers.
Fix a type conversion warning.
Add block transfer to null gpu.

For tests, better to have it here.
Merge pull request #6815 from unknownbrackets/gpu-minor

Eat cycles during GE block transfers