Move Attr helper methods to AttrHelpers trait to avoid to touch them from layout task.
Move Attr::local_name() to AttrHelpers/AttrHelpersForLayout.
Merge pull request #3274 from saneyuki/attr

Move Attr::set_value and Attr::value out of the Attr impl
Fix loading of file URLs with spaces
Merge pull request #3280 from metajack/path-with-spaces

Fix loading of file URLs with spaces
net: Premultiply alpha in PNGs.

Closes #2857.
Merge pull request #3281 from pcwalton/premultiply-alpha

net: Premultiply alpha in PNGs.
Partial fix for background images on wikipedia pages.

This fixes the visual artifacts seen at the top of wikipedia pages by:
 - Setting clipping rect to avoid images going outside their bounds.
 - Handling case of background-position being >= 100% such that wrapping is required.

However, the gradient is not currently visible on wikipedia. This relies on bug #1997 being fixed.