Move WebGLConstants from Renderer to Core

[lasalvavida]

Justification:

Currently we use WebGLConstants in Core (ComponentDataType, IndexDataType, PixelFormat, PrimitiveType, ...), but WebGLConstants lives in Renderer. Moving WebGLConstants to Core makes Core completely self contained with no dependencies on the rest of Cesium.

This PR also makes WebGLConstants public and updates the reference doc for it. We use WebGLConstants as part of the Cesium API in gltf-pipeline, so this seemed appropriate.

There were also a few members missing @memberof declarations in other modules which were being added to global in the doc, so this corrects those as well.

[pjcozzi]

@lasalvavida this is perfect, just one request: for maintenance and conciseness, it is perfectly OK to not document each individual enum vale in this case; instead, the reference doc for WebGLConstants can say that these match the WebGL spec constants and provide a link to the WebGL 1 and WebGL 2 specs.

[lasalvavida]

@lasalvavida this is perfect, just one request: for maintenance and conciseness, it is perfectly OK to not document each individual enum vale in this case; instead, the reference doc for WebGLConstants can say that these match the WebGL spec constants and provide a link to the WebGL 1 and WebGL 2 specs.

Okay. I need to see if I can make jsdoc hide the description column here for this. I added the comments because having a completely empty column looked a bit odd.

[lasalvavida]

This is updated removing the per entry comments and I've added links to the WebGL 1.0 and 2.0 specifications. I can't find a good way to hide the description column, so it is still there currently. Let me know if there are any other changes you would like to see, otherwise this is good to go.

[pjcozzi]

Thanks @lasalvavida!