QImageWriter: Fix default compressionratio

Before asking a image format handler to save an image, QImageWriter
sets the value of all supported options. For options like quality and
gamma, the default value is an illegal value (-1 and 0.0 resp.),
effectively telling the handler that the application has not requested
any particular value. But in the case of compressionratio, the default
was 0, a legal value. Fix by changing it to -1.

Change-Id: Iad6d4c2dbe269b25863e7d4967aa9ed2a7d1247f
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/gui/image/qimagewriter.cpp

@@ -264,7 +264,7 @@ QImageWriterPrivate::QImageWriterPrivate(QImageWriter *qq)
     deleteDevice = false;
     handler = 0;
     quality = -1;
-    compression = 0;
+    compression = -1;
     gamma = 0.0;
     optimizedWrite = false;
     progressiveScanWrite = false;