WebGLState: Implement true reset.

[Mugen87]

Related issue: see #20732 (comment)

Description

The implementation of WebGLState.state() is currently problematic. It resets flags but at the same time it does not reset the WebGL API to its original state.

The idea of this PR is to keep both in sync. If the flags are reset to their original values, same is done for the WebGL API now.

@jscastro76 @b1616 Can you please verify if this change would solve your issues?

[jscastro76]

Yes, in my case, it works. Shadows are back again even if context is shared with Mapbox.

Thanks @Mugen87! when this PR is aproved, if you want, you can answer the original question in SO and I'll accept it and vote it up.

[mrdoob]

Thanks!

[daumann]

With the latest version, it seems broken again.

[Mugen87]

The fiddle from the original issue still works with r132: https://jsfiddle.net/k7tw4boy/1/

It would be broken if no shadows are rendered.

[webglzhang]

I'm sorry that I haven't paid attention to #20732 for a long time due to work reasons. Let me explain the problem I'm trying to solve.I use the potree to do something, potree use the gl command to render pointCloud and use three.js to render mesh and backgroud. In the change the renderer，potree use the webglstate.reset to ensure the states of different renderers are isolated，but i found webglstate.reset is not do reset webgl state completely. And in this pr is fixed completely.👍