CALLs and JUMPs to misaligned addresses round down.

Fixes Pac-Man World 3.  Matches tests on hardware.
Note that VADDR/IADDR are not updated on PRIM.

Even though they do increase the addresses, they don't affect the CMD
registers.

Also, make sure not to use a crazy bytesRead for a zero-sized prim.
Pos=0 is treated the same as pos=1.
Merge pull request #6024 from unknownbrackets/gpu-minor2

Fix jump/call alignment, no-pos vertices