[test] Add OpenMPT ptoffset.mod test case

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix Protracker offset effect bug emulation

Correctly emulate Protracker offset bug (according to the OpenMPT test case)
by adding an extra variable to hold the original value.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Play MODs created with Scream Tracker in ST3 mode

MOD fingerprinting for Scream Tracker 3 seems to be reasonably accurate,
so use the ST3 event reader in this case.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Use Protracker quirk for invloop and fx9 bug emulation

Create a quirk for all Protracker-specific features such as the invloop
effect and offset bug emulation. Also set this quirk for MODs created
with accurate editors such as OpenMPT.

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