libmodplug: Add BWSB General Digital Music module loader

Based on patch by Alice Rowan (Lachesis) - see:
Konstanty/libmodplug#57

Author: sezero

CMakeLists.txt

@@ -102,6 +102,7 @@ if(SDLSOUND_DECODER_MODPLUG)
         src/libmodplug/load_mt2.c
         src/libmodplug/load_mtm.c
         src/libmodplug/load_okt.c
+        src/libmodplug/load_gdm.c
         src/libmodplug/load_psm.c
         src/libmodplug/load_ptm.c
         src/libmodplug/load_s3m.c

src/Makefile.os2

@@ -29,7 +29,7 @@ MODPSRCS = modplug.c sndfile.c fastmix.c snd_dsp.c snd_flt.c snd_fx.c sndmix.c &
            load_669.c load_amf.c load_ams.c load_dbm.c load_dmf.c load_dsm.c   &
            load_far.c load_it.c  load_mdl.c load_med.c load_mod.c load_mt2.c   &
            load_mtm.c load_okt.c load_psm.c load_ptm.c load_s3m.c load_stm.c   &
-           load_ult.c load_umx.c load_xm.c
+           load_gdm.c load_ult.c load_umx.c load_xm.c
 
 TIMISRCS = common.c instrum.c mix.c output.c playmidi.c readmidi.c resample.c  &
            tables.c timidity.c

src/SDL_sound_modplug.c

@@ -36,6 +36,7 @@ static const char *extensions_modplug[] =
     "DMF",   /* DMF DELUSION DIGITAL MUSIC FILEFORMAT (X-Tracker)           */
     "DSM",   /* DSIK Internal Format module                                 */
     "FAR",   /* Farandole module                                            */
+    "GDM",   /* General Digital Music                                       */
     "IT",    /* Impulse Tracker IT file                                     */
     "MDL",   /* DigiTracker module                                          */
     "MED",   /* OctaMed MED file                                            */

src/libmodplug/libmodplug.h

@@ -131,6 +131,7 @@ typedef const BYTE * LPCBYTE;
 #define MOD_TYPE_AMF0		0x200000
 #define MOD_TYPE_PSM		0x400000
 #define MOD_TYPE_J2B		0x800000
+#define MOD_TYPE_GDM		0x40000000 // Fake type
 #define MOD_TYPE_UMX		0x80000000 // Fake type
 #define MAX_MODTYPE		24
 
@@ -692,6 +693,7 @@ void delete_CSoundFile(CSoundFile *_this);
 	BOOL CSoundFile_ReadAMF(CSoundFile *_this, LPCBYTE lpStream, DWORD dwMemLength);
 	BOOL CSoundFile_ReadMT2(CSoundFile *_this, LPCBYTE lpStream, DWORD dwMemLength);
 	BOOL CSoundFile_ReadPSM(CSoundFile *_this, LPCBYTE lpStream, DWORD dwMemLength);
+	BOOL CSoundFile_ReadGDM(CSoundFile *_this, LPCBYTE lpStream, DWORD dwMemLength);
 	BOOL CSoundFile_ReadUMX(CSoundFile *_this, LPCBYTE lpStream, DWORD dwMemLength);
 
 	// MOD Convert function