This patch contains two isolated changes related to upcoming picture
caching improvements. Specifically:
* Determine the blit reason for stacking contexts with clips
earlier, during scene building. This simplifies the code and
allows better detection of redundant stacking contexts.
* Centralize the code for pushing batch instances into a small
number of places. This will simplify the switch to adding
a single primitive to multiple batch lists, in the case of
dirty regions with different batchers.
Differential Revision: https://phabricator.services.mozilla.com/D31898
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extra : moz-landing-system : lando