add internal texture to texture cache when creating a EnvironmentBRDF…

[TrevorDev]

…Texture for the scene

Repro: #8MGKWK#0
Run in console:

var ext = BABYLON.Engine.LastCreatedEngine._gl.getExtension('WEBGL_lose_context')
ext.loseContext();
setTimeout(()=>{
    ext.restoreContext()
}, 3000)