Fix XM pan slide memory

Regular XM pan slide effect has memory, but the volume column pan slide
doesn't. See the OpenMPT PanSlideMem.xm test case.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add OpenMPT PanSlideMem.xm test case

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>