libmodplug: Fix mLoopCount global setting

The module repeat count is supposed to be configured via mLoopCount
in the Modplug global settings. Setting this to a negative value
is supposed to enable looping forever (what game engines are most
likely to expect), but for some reason, this feature was *deliberately
broken* despite the default being 0. There's no reason for this to
be commented out.

Fixes Konstanty/libmodplug#51

See also:
#123
https://github.com/AliceLR/megazeux/blob/f83d897dc7df808ed4ad3eb31b5abb6a7b479d08/contrib/patches/libmodplug/01-libmodplug-0.8.9.0-fix-looping.diff

Mainstream pull request: Konstanty/libmodplug#102
Fixes: #123.

(cherry picked from commit 427281b)

Author: AliceLR

src/libmodplug/sndmix.c

@@ -219,7 +219,7 @@ BOOL CSoundFile_ProcessRow(CSoundFile *_this)
 				// End of song ?
 				if ((_this->m_nPattern == 0xFF) || (_this->m_nCurrentPattern >= MAX_ORDERS))
 				{
-					//if (!_this->m_nRepeatCount)
+					if (!_this->m_nRepeatCount)
 						return FALSE;     //never repeat entire song
 					if (!_this->m_nRestartPos)
 					{
@@ -253,7 +253,7 @@ BOOL CSoundFile_ProcessRow(CSoundFile *_this)
 							}
 						}
 					}
-//					if (_this->m_nRepeatCount > 0) _this->m_nRepeatCount--;
+					if (_this->m_nRepeatCount > 0) _this->m_nRepeatCount--;
 					_this->m_nCurrentPattern = _this->m_nRestartPos;
 					_this->m_nRow = 0;
 					if ((_this->Order[_this->m_nCurrentPattern] >= MAX_PATTERNS) || (!_this->Patterns[_this->Order[_this->m_nCurrentPattern]])) return FALSE;