Updates the requirements on [gltf](https://github.com/gltf-rs/gltf) to permit the latest version.
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/gltf-rs/gltf/blob/master/CHANGELOG.md">gltf's changelog</a>.</em></p>
<blockquote>
<h2>[0.16.0] - 2021-05-13</h2>
<h3>Added</h3>
<ul>
<li>Support for the <code>KHR_texture_transform</code> extension.</li>
<li>Support for the <code>KHR_materials_transmission_ior extension</code>.</li>
</ul>
<h3>Changed</h3>
<ul>
<li><code>Material::alpha_cutoff</code> is now optional.</li>
</ul>
<h3>Fixed</h3>
<ul>
<li>URIs with embedded data failing to import when using <code>import_slice</code>.</li>
<li>Serialization of empty primitives object being skipped.</li>
</ul>
<h2>[0.15.2] - 2020-03-29</h2>
<h3>Changed</h3>
<ul>
<li>All features are now exposed in the <a href="http://docs.rs/gltf">online documentation</a>.</li>
<li>Primary iterators now implement <code>Iterator::nth</code> explicitly for improved performance.</li>
</ul>
<h3>Fixed</h3>
<ul>
<li>Compiler warnings regarding deprecation of <code>std::error::Error::description</code>.</li>
</ul>
<h2>[0.15.1] - 2020-03-15</h2>
<h3>Added</h3>
<ul>
<li>New feature <code>guess_mime_type</code> which, as the name suggests, attempts to guess
the MIME type of an image if it doesn't exactly match the standard.</li>
</ul>
<h3>Changed</h3>
<ul>
<li><code>base64</code> updated to <code>0.11</code>.</li>
<li><code>byteorder</code> updated to <code>1.3</code>.</li>
<li><code>image</code> updated to <code>0.23.0</code>.</li>
<li><code>Format</code> has additional variants for 16-bit pixel formats.</li>
</ul>
<h3>Fixed</h3>
<ul>
<li>Off-by-one error when reading whole files incurring a gratuitous reallocation.</li>
</ul>
<h2>[0.15.0] - 2020-01-18</h2>
<h3>Added</h3>
<ul>
<li>Support for the <code>KHR_materials_unlit</code> extension, which adds an <code>unlit</code> field</li>
</ul>
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</blockquote>
<p>... (truncated)</p>
</details>
<details>
<summary>Commits</summary>
<ul>
<li>See full diff in <a href="https://github.com/gltf-rs/gltf/commits">compare view</a></li>
</ul>
</details>
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</details>
make more information available from loaded GLTF model
* make gltf nodes available as assets
* add list of primitive per mesh, and their associated material
* complete gltf structure
* get names of gltf assets
* only load materials once
* add labels with node names
