wbmp conversion warning fix.

ext/gd/gd.c

@@ -1721,7 +1721,9 @@ static void _php_image_output(INTERNAL_FUNCTION_PARAMETERS, int image_type, char
 
 		switch (image_type) {
 			case PHP_GDIMG_CONVERT_WBM:
-				if (q < 0 || q > 255) {
+				if (q == -1) {
+					q = 0;
+				} else if (q < 0 || q > 255) {
 					php_error_docref(NULL TSRMLS_CC, E_WARNING, "Invalid threshold value '%d'. It must be between 0 and 255", q);
 					q = 0;
 				}
@@ -1766,9 +1768,12 @@ static void _php_image_output(INTERNAL_FUNCTION_PARAMETERS, int image_type, char
 
 		switch (image_type) {
 			case PHP_GDIMG_CONVERT_WBM:
-				if (q < 0 || q > 255) {
-					php_error_docref(NULL TSRMLS_CC, E_WARNING, "Invalid threshold value '%d'. It must be between 0 and 255", q);
-				}
+ 				if (q == -1) {
+  					q = 0;
+  				} else if (q < 0 || q > 255) {
+  					php_error_docref(NULL TSRMLS_CC, E_WARNING, "Invalid threshold value '%d'. It must be between 0 and 255", q);
+ 					q = 0;
+  				}
 				gdImageWBMP(im, q, tmp);
 				break;
 			case PHP_GDIMG_TYPE_JPG: