Do not clear when rendering to target in PostProcessEffect

[felixpalmer]

For #8703

Background

Currently the PostProcessEffect clears the target it is drawing to. In order to implement per-layer postprocessing we need to disable this clearing

Change List

• Add clear option to ScreenPass and set to false when rendering to target in PostProcessEffect

[coveralls]

coverage: 89.976% (+0.001%) from 89.975%
when pulling 3384670 on felix/post-process-clear
into 51a3a3c on master.

[Pessimistress]

If deck is going to support per-view or per-layer effects, I don't think this option should be set by the user. Clearing of the target should be disabled in PostProcessEffect always, and handled in DeckRenderer instead.

[felixpalmer]

@Pessimistress I've removed the user visible option. From my testing it doesn't seem that we need the explicit clear in deck-renderer as the canvas is cleared on every frame automatically

[Pessimistress]

The canvas is auto cleared because preserveDrawingBuffer defaults to false. You should not rely on it, though.

[felixpalmer]

@Pessimistress updated to explicitly clear & tested with a canvas with preserveDrawingBuffer=true