Block layout RPC on the first layout call
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.
remove unnecessary proc from resource loader factory
create about_loader, handle about:blank
Add -v or --verbose to mach build
Merge pull request #3291 from EdorianDark/master

Add -v or --verbose to mach build
Merge pull request #3216 from seanmonstar/about-blank

Implements about:blank in an about_loader
Fix reftest now that the background image code clips correctly.
Merge pull request #3254 from cgaebel/issue-3232

Block LayoutRPC until the first reflow.
Merge pull request #3283 from glennw/bg-image

Partial fix for background images on wikipedia pages.
Reduce number of font instances and shaper structures created.

The font cache previously kept weak references to the fonts,
however in layout these fonts are dropped before they are
referenced again, so no caching was being used.

For now, just hold a strong reference in the cache, which means
that fonts will never be thrown out (this will need to be fixed
in the future).
Make use of the list of Atoms in ClassSelector

Make use of the list of Atoms in the class attribute selector
(ClassSelector) in selector_matching.

Fixes #3111
Fix comment and and reference to bug #3300
Print debug message instead of fail when unable to load web font. Fixes #3301.
Coalesce pending reflow messages so they don't queue up.
Merge pull request #3294 from glennw/guardian-layout

Coalesce pending reflow messages so they don't queue up.
Merge pull request #3213 from gilles-leblanc/issue-3111

Make use of the list of Atoms in the class attribute in selector matchin...
Merge pull request #3304 from glennw/allow-font-failure

Print debug message instead of fail when unable to load web font. Fixes #3301.
Store Compositor ReadyState per-pipeline

Instead of storing a single ReadyState, store one per pipeline and
track the earliest one.
Store one RenderState per-pipeline in the Compositor

This can later be used to decide whether the entire pipeline is ready
for rendering.