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 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.
auto merge of #1260 : metajack/servo/kmcallister-rustpkg, r=metajack,me

Rebased from @kmcallister's branch to fix mac platform issues.
submodule updated
Remove unused files
auto merge of #1291 : sammykim/servo/unused-file, r=jdm
auto merge of #1288 : ryanhc/servo/transparent_png, r=larsbergstrom

Modified to use libpng to read .png files. We use stb_image to read all other image files.
This requires updated submodule: https://github.com/mozilla-servo/rust-png/pull/13

This patch is for:
https://github.com/mozilla/servo/issues/1279
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
auto merge of #1292 : metajack/servo/not-parallel, r=larsbergstrom

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
auto merge of #1293 : SimonSapin/servo/root, r=SimonSapin

Rebase/squash of @therealglazou’s #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.
auto merge of #1294 : metajack/servo/isolate-backup, r=larsbergstrom

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.