QImageWriter: Detect failure due to trying to write a QImage()

This is not a bug in QImageWriter, but caller code. We should be
explicit about what the problem was so it can be fixed.

[ChangeLog][QtGui][QImageWriter] Add QImageWriter::InvalidImageError to
communicate invalid attempts to write a bad QImage (for instance, a null
QImage).

Change-Id: I0333b8263f1da1c672bed17dab48bfd6cafe41a2
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/gui/image/qimagewriter.h b/src/gui/image/qimagewriter.h
index 58f8c51472..fd1fdd07e8 100644
--- a/src/gui/image/qimagewriter.h
+++ b/src/gui/image/qimagewriter.h
@@ -60,7 +60,8 @@ public:
     enum ImageWriterError {
         UnknownError,
         DeviceError,
-        UnsupportedFormatError
+        UnsupportedFormatError,
+        InvalidImageError
     };
 
     QImageWriter();