Auto merge of #20214 - mrobinson:stop-using-using-localclip-roundedrect, r=glennw

Stop using LocalClip::RoundedRect

We would like to remove this functionality from WebRender, so convert
its use to clip scroll nodes. This change also removes the redundant
BlocBlockStackingContextType in favor of Option<StackingContextType>,
which is just as expressive. This simplifies the code a bit.

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