Fix signed comparison

Author: Rangi42

src/gfx/process.cpp

@@ -226,8 +226,8 @@ class Png {
 		if (options.inputSlice.height == 0 && height % 8 != 0) {
 			fatal("Image height (%" PRIu32 " pixels) is not a multiple of 8!", height);
 		}
-		if (options.inputSlice.left + options.inputSlice.width * 8 > width
-		    || options.inputSlice.top + options.inputSlice.height * 8 > height) {
+		if (options.inputSlice.width * 8 > width - options.inputSlice.left
+		    || options.inputSlice.height * 8 > height - options.inputSlice.top) {
 			error(
 			    "Image slice ((%" PRIu32 ", %" PRIu32 ") to (%" PRId32 ", %" PRId32
 			    ")) is outside the image bounds (%" PRIu32 "x%" PRIu32 ")!",