Fix for #11160 (Wrong PNG/JPEG resolution on linux)

[sergiuiscoding]

Fix png/jpg resolution metadata which caused the images to render stretched.

[ariya]

Without more explanation why this unifies the resolution on Linux and Windows, I don't understand its purpose.

[sergiuiscoding]

From what I've found out (see #11160) the rendered image has some messed-up metadata (specifically x res / y res) which causes the image to be rendered with wrong aspect ratio (aka stretched) by some viewers (Windows Photo Viewer).
By setting "dotsPerMeter" we correct the resolution / image aspect ratio, so it's rendered consistently across viewers/platforms.

[ariya]

Sure, it does explain what the fix does. However, I'm still baffled as to why QImage produces different resolution in Linux vs other platforms.

[sergiuiscoding]

I see. I've reduced the rendering code to just a few lines (by the book, disabling the tile-based rendering), so I'm guessing the problem is somewhere in QWebFrame::render() or somewhere down the road. Unfortunately I don't have time to dig on this lead. I badly needed a fix for this, so setting the dpm did the trick.

[ariya]

Fair enough, but let's postpone landing the fix until we really know what causes the issue. Also, with our rendering-to-base64 feature, this should be tested as well to prevent future regressions.

[sergiuiscoding]

Closing as the issue has been fixed in the 1.9 release (3f42fb2)