WebGLRenderer#clearBeforeRender

This sets if the WebGLRenderer will clear the context texture or not before the new render pass. If true:
If the Stage is NOT transparent, Pixi will clear to alpha (0, 0, 0, 0).
If the Stage is transparent, Pixi will clear to the target Stage's background color.
Disable this by setting this to false. For example: if your game has a canvas filling background image, you often don't need this set.