gfx: Rewrite display list construction to make stacking-contexts more
first-class.

This implements the scheme described here:

    https://groups.google.com/forum/#!topic/mozilla.dev.servo/sZVPSfPVfkg

This commit changes Servo to generate one display list per stacking
context instead of one display list per layer. This is purely a
refactoring; there are no functional changes. Performance is essentially
the same as before. However, there should be numerous future benefits
that this is intended to allow for:

* It makes the code simpler to understand because the "new layer needed"
  vs. "no new layer needed" code paths are more consolidated.

* It makes it easy to support CSS properties that did not fit into our
  previous flat display list model (without unconditionally layerizing
  them):

  o `opacity` should be easy to support because the stacking context
    provides the higher-level grouping of display items to which opacity
    is to be applied.

  o `transform` can be easily supported because the stacking context
    provides a place to stash the transformation matrix. This has the side
    benefit of nicely separating the transformation matrix from the
    clipping regions.

* The `flatten` logic is now O(1) instead of O(n) and now only needs to
  be invoked for pseudo-stacking contexts (right now: just floats),
  instead of for every stacking context.

* Layers are now a proper tree instead of a flat list as far as layout
  is concerned, bringing us closer to a production-quality
  compositing/layers framework.

* This commit opens the door to incremental display list construction at
  the level of stacking contexts.

Future performance improvements could come from optimizing allocation of
display list items, and, of course, incremental display list
construction.
Including webstorage folder in tests
Correct the checks in step 5 of Node::pre_insert.
auto merge of #4003 : Ms2ger/servo/pre-insert, r=jdm
auto merge of #4005 : nkdalmia/servo/master, r=jdm

This will enable the web platform test in the webstorage folder.
Update gleam submodule, and use glfw to load OpenGL function pointers.
auto merge of #4011 : glennw/servo/gleam-update, r=larsbergstrom
auto merge of #3990 : pcwalton/servo/stacking-contexts, r=glennw

This implements the scheme described here:

    https://groups.google.com/forum/#!topic/mozilla.dev.servo/sZVPSfPVfkg

This commit changes Servo to generate one display list per stacking
context instead of one display list per layer. This is purely a
refactoring; there are no functional changes. Performance is essentially
the same as before. However, there should be numerous future benefits
that this is intended to allow for:

* It makes the code simpler to understand because the "new layer needed"
  vs. "no new layer needed" code paths are more consolidated.

* It makes it easy to support CSS properties that did not fit into our
  previous flat display list model (without unconditionally layerizing
  them):

  o `opacity` should be easy to support because the stacking context
    provides the higher-level grouping of display items to which opacity
    is to be applied.

  o `transform` can be easily supported because the stacking context
    provides a place to stash the transformation matrix. This has the side
    benefit of nicely separating the transformation matrix from the
    clipping regions.

* The `flatten` logic is now O(1) instead of O(n) and now only needs to
  be invoked for pseudo-stacking contexts (right now: just floats),
  instead of for every stacking context.

* Layers are now a proper tree instead of a flat list as far as layout
  is concerned, bringing us closer to a production-quality
  compositing/layers framework.

* This commit opens the door to incremental display list construction at
  the level of stacking contexts.

Future performance improvements could come from optimizing allocation of
display list items, and, of course, incremental display list
construction.

r? @glennw
f? @mrobinson @cgaebel
Update green-rs submodule to get android tls fix.
Redirect android resource folder (user-agent.css and friends) to /sdcard/servo. This is a temporary solution, until they are packaged properly.
auto merge of #4013 : glennw/servo/android-resources, r=larsbergstrom

This is a temporary solution, until they are packaged properly.
Add missing gfx dependency, that was causing a race in cargo on the build machine.
auto merge of #4012 : glennw/servo/update-green-rs, r=mbrubeck
key[board]event args for Document#createEvent
auto merge of #4014 : wenderen/servo/doc-createevent-keyevents, r=Manishearth

fix #4007
Initial attempt at rerolling patch from PR-2610
Change previous statics into consts.
Amend names accordingly as they should be uppercase.
Add new options to FlowFlags, and expand it from u8 to u16 to accomodate.
Remove trailing whitespace