WebKitGTK+ 2.15.4 released!

This is a development release leading toward 2.16 series.

Make accelerating compositing mode on-demand again. By default it will only be used for websites that require it, saving a lot of memory on websites that don’t need it.
Add API to manage hardware acceleration policy.
Enable CSS Grid Layout by default.
Add API to create ephemeral WebViews to replace the legacy private browsing setting that is now deprecated.
Handle HTTP authentication for downloads having a WebView associated.
Add API to WebKitWebsiteDataManager to handle websites data.
Fix BadDamage X errors happening when resizing the WebView.
Fix several crashes and rendering issues.

Thanks to all the contributors who made possible this release.