Use the "unused" bit in SDL palettes for alpha.

stepmania/src/RageDisplay_D3D.cpp

@@ -972,7 +972,7 @@ unsigned RageDisplay_D3D::CreateTexture(
 			pal.p[i].peGreen = c.g;
 			pal.p[i].peBlue = c.b;
 			bool bIsColorKey = img->flags & SDL_SRCCOLORKEY && (unsigned)i == img->format->colorkey;
-			pal.p[i].peFlags = bIsColorKey ? 0x00 : 0xFF;
+			pal.p[i].peFlags = bIsColorKey ? BYTE(0x00) : c.unused;
 		}
 
 		ASSERT( g_TexResourceToTexturePalette.find(uTexHandle) == g_TexResourceToTexturePalette.end() );

stepmania/src/RageDisplay_OGL.cpp

@@ -1127,7 +1127,7 @@ unsigned RageDisplay_OGL::CreateTexture(
 			if(img->flags & SDL_SRCCOLORKEY && i == int(img->format->colorkey))
 				palette[p++] = 0;
 			else
-				palette[p++] = 0xFF; /* opaque */
+				palette[p++] = img->format->palette->colors[i].unused;
 		}
 
 		/* Set the palette. */