New WebGL warning in recent Firefox versions

Sandcastle example: any Sandcastle

Browser: Firefox 70+

Operating System: Windows

The latest Firefox has started spewing multiple copies of the following into the console:

> Error: WebGL warning: generateMipmap: Tex image TEXTURE_2D level 0 is incurring lazy initialization.

It points to [this line in Texture#generateMipmap](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Renderer/Texture.js#L679).

Start with Source/Renderer/Texture.js at line 679 and reproduce the warning in a Sandcastle example using Firefox 70+ on Windows. Compare the console output around Texture#generateMipmap; done means the repeated lazy-initialization warning is no longer produced in the reported scenario.

Written by the indexing model from the issue text.