clutter: Drop the no-clear stage hint

Carlos Garnacho requested to merge wip/carlosg/drop-no-clear-hint into master

A compositor is notably opaque (usually has nothing to be painted on!). gnome-shell sets this hint, but there's no reason why we wouldn't want it by default.

Also, the color buffer being cleared messes with stencil clips, as the clear operation by definition ignores the stencil buffer. We want to use these more extensively in the future, so just drop this API.

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