Engine.dispose throws "this.releaseComputeEffects is not a function"

In 5.0.0-alpha.21, when we call Engine.dispose() it throws “TypeError: this.releaseComputeEffects is not a function”. Looks like ThinEngine.dispose() unconditionally calls releaseComputeEffects() even though that will only be defined if you’ve loaded engine.computeShader.ts.

just found the same issue in my project

This PR will fix the problem: