Accept embedded gltf files as _meshfile_objects

[SeanCurtis-TRI]

This accepts embedded .gltf files as _meshfile_objects. It may also accept distributed gltf files as long as the meshcat server respects meshcat's requests for the missing .bin and image files.

This cannot be part of the ExtensibleObjectLoader (EOL). EOL inhertis from ObjectLoader where as GLTFLoader inherits from Loader. The workflow of ObjectLoader is incompatible with GLTFLoader's workflow. Fortunately, GLTFLoader produces a THREE.Group that we can simply stash in the scene.

To support PBR materials (reflective surfaces which reflect an environment map), we change the "background" gradient to support environment textures. This is largely an innocuous change. However, there is a distinct difference between the new background and the old.

In the old, the gradient was registered in screen space. Whether I was looking up or down, the top color was top of the screen and the bottom at the bottom of the screen.

The new environment map now use the "top" (+z infinitely far away) and bottom (-z infinitely far away) of the world. So, rotating the camera to look up sees the upper pole and looking down shows the lower pole.

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This change is 

[SeanCurtis-TRI]

Here are a couple of screen grabs for what this enables (taken from a hacked version of Drake's meshcat_manual_test.cc).

Simple, abstract cube with a non-trivial material.

The canonical "damaged helmet" as featured on Khronos's site.

[jwnimmer-tri]

\CC @pathammer in case you have any suggestions

[RussTedrake]

Thanks @pathammer ! Very helpful.

@SeanCurtis-TRI -- when I run the new test (thank you for adding it), the rendering looks fine by my browser java console warns

main.min.js:2 THREE.WebGLTextures: Unsupported texture encoding: srgb-linear
H @ main.min.js:2
k @ main.min.js:2
R @ main.min.js:2
Bo @ main.min.js:2
da.upload @ main.min.js:2
(anonymous) @ main.min.js:2
renderBufferDirect @ main.min.js:2
Oe @ main.min.js:2
De @ main.min.js:2
Ie @ main.min.js:2
render @ main.min.js:2
update @ main.min.js:2
fromEquirectangularTexture @ main.min.js:2
get @ main.min.js:2
render @ main.min.js:2
render @ main.min.js:2
render @ main.min.js:2
animate @ main.min.js:2
Viewer @ main.min.js:2
(anonymous) @ meshfile_object_gltf.html:13

Should I be worried?

[SeanCurtis-TRI]

Thanks, @RussTedrake.