Add breakpoint utility function.
Store content offset in unscaled pixels

Content offset is now stored in unscaled pixels, so that it can be
applied to tile positions during compositor rendering without unscaling
it. This means that when using it in calculations of layer size (stored
in device pixels), we need to scale it. Scene scale is now stored as a
scale rather than a transformation.

Fixes #3269.
Fixes #3042.
Add unrooted_must_root lint for enums and structs containing JS<T>, as well as functions with JS<T> in their parameter list

For safe wrappers over JS<T> (eg Temporary<T>) use #[allow(unrooted_must_root)].
For all other types containing a #[must_root] value, annotate the type with #[must_root] to ensure that it is never used unrooted
Removed unused BlobMethods and CommentMethods traits, fixes #3366
Add unrooted_must_root lint for usages of JS<T> in let/for bindings
Unwrap pointers and miscellany for unrooted_must_root check
Address review comments
Make Reflector #[must_root], propagate to non-HTMLElements
Use #[must_root] for HTMLElements
Merge pull request #3318 from mrobinson/scrolling

Store content offset in unscaled pixels
Update README.md
Update README.md
Merge pull request #3377 from LalehB/patch-5

Update README.md
Merge pull request #3373 from ProgramFOX/issue3366

Removed unused BlobMethods and CommentMethods traits, fixes #3366
Fix `mach cargo` is missing
Merge pull request #3367 from jdm/breakpoint

Add breakpoint utility function.
Merge pull request #3382 from saneyuki/mach

Fix `mach cargo` is missing
Merge pull request #3374 from Manishearth/lint_unrooted_jsmanaged

Add lint for ensuring proper rooting of JS<T>; r=jdm