Local rectangle clipping for transformed primitives

Finishes servo#427 implementation, now the text is cut evenly and not aligned to tile borders.
Also refactored `build_bounding_rect()` as well as a bit of `assign_prims_to_screen_tiles()`.

![textrun-comparison3](https://cloud.githubusercontent.com/assets/107301/19860234/5ce0c85a-9f5e-11e6-82f2-d72f19e4641a.png)

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…jang

Bump cbindgen from 0.14.2 to 0.14.6

Bumps [cbindgen](https://github.com/eqrion/cbindgen) from 0.14.2 to 0.14.6.
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/eqrion/cbindgen/blob/master/CHANGES">cbindgen's changelog</a>.</em></p>
<blockquote>
<h2>0.14.6</h2>
<pre><code> * Fixed the builds with older versions of rustc.
</code></pre>
<h2>0.14.5</h2>
<pre><code> * Add support to specify line ending style ([#568](mozilla/cbindgen#568))
 * Add cbindgen:ptrs-as-arrays annotation to allow making function
   arguments C/C++ arrays.
</code></pre>
<h2>0.14.4</h2>
<pre><code> * Allow to override the mangling separator ([#502](mozilla/cbindgen#502))
<ul>
<li>
<p>cbindgen now handles better having ZSTs in template parameters, and
default template parameters (<a href="https://github-redirect.dependabot.com/eqrion/cbindgen/issues/563">#563</a>).</p>
</li>
<li>
<p>Support for annotating nonnull pointers (<a href="https://github-redirect.dependabot.com/eqrion/cbindgen/issues/558">#558</a>)</p>
</li>
<li>
<p>Fixed bitflags that overflow a signed integer (<a href="https://github-redirect.dependabot.com/eqrion/cbindgen/issues/556">#556</a>)</p>
</li>
<li>
<p>Support for wildcard argument names (<a href="https://github-redirect.dependabot.com/eqrion/cbindgen/issues/550">#550</a>)</p>
</li>
<li>
<p>Support for the never return type, with configurable annotation (<a href="https://github-redirect.dependabot.com/eqrion/cbindgen/issues/549">#549</a>)</p>
</li>
<li>
<p>Properly reject arrays as function arguments (<a href="https://github-redirect.dependabot.com/eqrion/cbindgen/issues/540">#540</a>)
</code></pre></p>
</li>
</ul>
<h2>0.14.3</h2>
<pre><code> * Introduce cbindgen:ignore comment annotation, to allow ignoring items or modules. ([#519](mozilla/cbindgen#519))
 * Support for casts in constant expressions. ([#526](mozilla/cbindgen#526))
 * Make a non-fatal error a warning message. ([#535](mozilla/cbindgen#535))
 * Add a --metadata option to the CLI, to allow passing pre-computed cargo metadata. ([#538](mozilla/cbindgen#538))
</code></pre>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/eqrion/cbindgen/commit/e4da7d39a6d1481420440b37dcfe85d7ea4527d5"><code>e4da7d3</code></a> v0.14.6</li>
<li><a href="https://github.com/eqrion/cbindgen/commit/1e46e53ae22b1d85bc794911dd750543e374bad7"><code>1e46e53</code></a> v0.14.5</li>
<li><a href="https://github.com/eqrion/cbindgen/commit/fe656442a562ad1c7c19706d7b068492ecf58230"><code>fe65644</code></a> Document ptrs-as-arrays annotation</li>
<li><a href="https://github.com/eqrion/cbindgen/commit/07c3aed03e6a3b53a580f8c6cfc4c9ea446104bb"><code>07c3aed</code></a> Add ptrs-as-arrays tests</li>
<li><a href="https://github.com/eqrion/cbindgen/commit/ee4638ab10f4a7dcf9a8abd3b5fc2e2463dfcde5"><code>ee4638a</code></a> Add an annotation to represent pointers as arrays</li>
<li><a href="https://github.com/eqrion/cbindgen/commit/73ea04c2b0f8926ebb1491ab311e591721d5dbd2"><code>73ea04c</code></a> Add badge to README</li>
<li><a href="https://github.com/eqrion/cbindgen/commit/6b56082be8403c9dfe237cf0d4274369b7a83cd9"><code>6b56082</code></a> Replace travis with Github Actions</li>
<li><a href="https://github.com/eqrion/cbindgen/commit/fec6bdad8cd9a0d37d506a6033ae3ee21c4f6989"><code>fec6bda</code></a> Fix clippy warnings</li>
<li><a href="https://github.com/eqrion/cbindgen/commit/3ca0e75ed906ae505740657647b08e04cd844f14"><code>3ca0e75</code></a> Add support for specifying line ending style (<a href="https://github-redirect.dependabot.com/eqrion/cbindgen/issues/568">#568</a>)</li>
<li><a href="https://github.com/eqrion/cbindgen/commit/83cdbd897f02eaecb090a3f216899dcc14b43959"><code>83cdbd8</code></a> Release v.0.14.4</li>
<li>Additional commits viewable in <a href="https://github.com/eqrion/cbindgen/compare/v0.14.2...v0.14.6">compare view</a></li>
</ul>
</details>
<br />

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