Fix environment switch glitches

[MortimerGoro]

Fixes #1021

The problem is that TextureCubemap internal initilization is triggered from two threads at the same time, the loader thread, and the main render thread when it binds the cubemap texture for the current frame. This happens because we are using a geometry already added to the scenegraph after a environment change.

[philip-lamb]

Tested OK on Vive Focus, Oculus Go.

[cvan]

👍 looks fixed, thanks! I tested switching environments and installing a custom environment on both the Pico G2 and Oculus Go. environments appear to load significantly faster now (than before this branch) on the Pico G2 - nice work.

[bluemarvin]

environments appear to load significantly faster on the Pico G2 - nice work.

The Pico G2 CPU is significantly faster than the Go.

[cvan]

environments appear to load significantly faster on the Pico G2 - nice work.

The Pico G2 CPU is significantly faster than the Go.

my phrasing was ambiguous: before this PR, the environments changed faster on the Oculus Go than on the Pico G2. in this branch, the environments load at perceptibly the same speed. good improvements across the board 👍

[MortimerGoro]

before this PR, the environments changed faster on the Oculus Go than on the Pico G2. in this branch, the environments load at perceptibly the same speed. good improvements across the board

It makes sense, the environment was loaded twice due to the initialization race condition