fix crashy bugs in snes gfx debugger

BizHawk.Client.EmuHawk/tools/SNES/SNESGraphicsDebugger.cs

@@ -1160,6 +1160,10 @@ namespace BizHawk.Client.EmuHawk
 					{
 						var bg = si.BG[(int)CurrDisplaySelection];
 
+						//unavailable BG for this mode
+						if (bg.Bpp == 0)
+							break;
+
 						if (bg.TileSize == 16) { tx /= 2; ty /= 2; } //worry about this later. need to pass a different flag into `currViewingTile`
 
 						int tloc = ty * bg.ScreenSizeInTiles.Width + tx;

BizHawk.Emulation.Cores/Consoles/Nintendo/SNES/SNESGraphicsDecoder.cs

@@ -6,8 +6,7 @@
 //http://wiki.superfamicom.org/snes/show/Registers
 
 //TODO
-//SF2 title art doesnt seem to show up.. 
-//scanline control doesnt work?
+//when a BG is not available, the last rendered BG still shows up. should clear it
 
 using System;
 
@@ -645,6 +644,9 @@ namespace BizHawk.Emulation.Cores.Nintendo.SNES
 		/// </summary>
 		public void DecodeBG(int* screen, int stride, TileEntry[] map, int tiledataBaseAddr, ScreenSize size, int bpp, int tilesize, int paletteStart)
 		{
+			//emergency backstop. this can only happen if we're displaying an unavailable BG or other similar such value
+			if (bpp == 0) return;
+
 			int ncolors = 1 << bpp;
 
 			int[] tileBuf = new int[16*16];