Build some dependencies with rustpkg

Also take a rust-layers bugfix.

In collaboration with Jack Moffitt.
Pass Mac OS version cfg flags to rust-opengles build

This was in the rust-opengles Makefile.in pre-rustpkg.
Update rust-alert to find deps through rustpkg
Update mac platform submodules.
Add acid2 test case.
add support for transparant png
Fix clean-servo to include clean-style.
Update dependencies.
Port gfx crate to new CF bindings.
Set cfg flags on all rust packages.
submodule updated
Remove unused files
Disable parallel builds at top level.

Sub-builds are still parallelized. This is a workaround for rustpkg
issues. See https://github.com/mozilla/rust/issues/9650
Implement :root pseudo-class. Fix #1263, fix #1265
Move backup related stuff to its own Makefile.

This will prevent Makefile errors from affecting backups. This happens when
different branches include Makefile helpers other branches don't have. This
should be more reliable.
fix indent and format
[Not Complete] it has scroll request timing issue