gpgx: change lowpass range to ushort to make at least some sense to user.

BizHawk.Emulation.Cores/Consoles/Sega/gpgx/GPGX.ISettable.cs

@@ -81,8 +81,8 @@ namespace BizHawk.Emulation.Cores.Consoles.Sega.gpgx
 
 			[DisplayName("Low Pass Range")]
 			[Description("Only active when filter type is lowpass")]
-			[DefaultValue((short)-26215)]
+			[DefaultValue((ushort)39321)]
-			public short LowPassRange { get; set; }
+			public ushort LowPassRange { get; set; }
 
 			[DisplayName("Three band low cutoff")]
 			[Description("Only active when filter type is three band")]

BizHawk.Emulation.Cores/Consoles/Sega/gpgx/LibGPGX.cs

@@ -39,7 +39,7 @@ namespace BizHawk.Emulation.Cores.Consoles.Sega.gpgx
 				ThreeBand = 2
 			}
 			public FilterType Filter;
-			public short LowPassRange;
+			public ushort LowPassRange;
 			public short LowFreq;
 			public short HighFreq;
 			public short LowGain;

genplus-gx/cinterface/cinterface.c

@@ -486,7 +486,7 @@ GPGX_EX void gpgx_get_sram(void **area, int *size)
 struct InitSettings
 {
 	uint8_t Filter;
-	int16_t LowPassRange;
+	uint16_t LowPassRange;
 	int16_t LowFreq;
 	int16_t HighFreq;
 	int16_t LowGain;

genplus-gx/libretro/osd.h

@@ -45,7 +45,7 @@ struct
   uint8 mono;
   int16 psg_preamp;
   int16 fm_preamp;
-  int16 lp_range;
+  uint16 lp_range;
   int16 low_freq;
   int16 high_freq;
   int16 lg;