Fix #304: test suite failure in gif/bug00006 [2.2.3]

gdImageGif() converts to palette using GD_QUANT_DEFAULT which is
configuration dependend. To circumvent this we're converting to palette
ourselves, explicitly setting the desired quantization method.

tests/gif/bug00006.c

@@ -27,6 +27,10 @@ int main()
 		gdTestErrorMsg("Cannot open <%s> for writing\n", file);
 		return 1;
 	}
+
+	gdImageTrueColorToPaletteSetMethod(im, GD_QUANT_JQUANT, 0);
+	gdImageTrueColorToPalette(im, 1, 256);
+
 	gdImageGif(im,fp);
 	fclose(fp);