Don't strip minus sign on the first frame of an arpeggio envelope

[pinobatch]

The first frame of an arpeggio envelope is written as its absolute value, causing arpeggios that rise toward the note's pitch (such as Hummer Team stuff) to sound wrong.

Steps to reproduce (change paths accordingly):

~/".wine/drive_c/Program Files (x86)/FamiTracker/FamiTracker.exe" leading_neg_arp.ftm -export test.txt
../ft2p -i test.txt -o test.pently
less test.pently
~/".wine/drive_c/Program Files (x86)/FamiTracker/FamiTracker.exe" leading_neg_arp_looped.ftm -export test2.txt
../ft2p -i test2.txt -o test2.pently
less test2.pently

Result: Each chord_ instrument begins with a positive pitch. For example:

pitch 7 -7 -3 -3 0 0 -7 -7 -3 -3 0

Expected: Each chord_ instrument begins with a negative pitch. For example:

pitch -7 -7 -3 -3 0 0 -7 -7 -3 -3 0

leading_neg_arp.zip