script: Fix double borrow error when going back. Closes #4402.

I have verified that back and forward work once again.
Fix some writing mode mismatch assertions errors.

The rendering is still wrong beause of #2795, but at least we get a rendering.

(This test change is just for readability, it should be equivalent to before.)
script: Remove glob imports added in #4405
layout: Implement `clip` per CSS 2.1 § 11.1.2.

Only the recommended, comma-separated syntax is supported.
auto merge of #4394 : pcwalton/servo/clip, r=SimonSapin

Only the recommended, comma-separated syntax is supported.

r? @SimonSapin
auto merge of #4403 : pcwalton/servo/back-borrow, r=jdm

I have verified that back and forward work once again.

Sorry about this one. r? @jdm
auto merge of #4425 : SimonSapin/servo/mixed-writing-modes, r=mbrubeck

The rendering is still wrong beause of #2795, but at least we get a rendering.

(This test change is just for readability, it should be equivalent to before.)

r? @mbrubeck
auto merge of #4434 : saneyuki/servo/glob_script, r=jdm

#4406