layout: Make `box-sizing` only affect explicitly-specified or percentage
sizes, not automatically computed ones.

Improves GitHub significantly.
fix debian install instructions
Disable html/semantics/forms/the-input-element/type-change-state.html.ini
auto merge of #3656 : Manishearth/servo/disable-type-change, r=Ms2ger

I haven't yet gotten to the root of this, but it's breaking the other PRs.
auto merge of #3615 : pcwalton/servo/box-sizing-fix, r=glennw

Improves GitHub significantly.

r? @glennw
auto merge of #3627 : steveklabnik/servo/master, r=larsbergstrom

This may be because I'm on Ubuntu, but this was the name of the package.
Include leading margin in inline block size

According to the documentation for Fragment::position, the inline axis
should include margin size, so we include it for blocks. Also fix
place_float which assumed that it was not included and
assign_inline_sizes which overrode the size set in
set_inline_size_constraint_solutions.

Typically this issue was hidden by large tile sizes, but fitted tiles
makes it more common.
auto merge of #3618 : mrobinson/servo/layer-sizing, r=pcwalton

Extra size from margins should be included in block size, so that
layers are large enough to include the entire block. This is typically
hidden by large tile sizes (512x512), but fitted tiles makes the issue
a lot more common.
layout: Rewrite clipping to be per-display-item instead of having
a separate `ClipDisplayItem`.

We push down clipping areas during absolute position calculation. This
makes display items into a flat list, improving cache locality. It
dramatically simplifies the code all around.

Because we need to push down clip rects even for absolutely-positioned
children of non-absolutely-positioned flows, this patch alters the
parallel traversal to compute absolute positions for
absolutely-positioned children at the same time it computes absolute
positions for other children. This doesn't seem to break anything either
in theory (since the overall order remains correct) or in practice. It
simplifies the parallel traversal code quite a bit.

See the relevant Gecko bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=615734