The clip-path SVG property was conditionally set in thumbnail.js and
also conditionally set or removed in renderer.js. This patch refactors
the logic to occur in a single place, renderer.js.
The refactor was made with the following considerations:
• The one condition under which thumbnail.js would set clip-path was,
given a thumbnail exists, the thumbnail was not tall and clip-path
would be set to `url(#mwe-popups-mask)`. Otherwise, thumbnail.js would
not set clip-path.
• The logic in renderer.js for setting the attribute doesn't change
since overwriting the clip-path is equivalent to not having a
preexisting value. The case for removing the attribute itself is
replaced by inverting the condition, `flippedY`, and combining it with
the thumbnail.js condition, `!isTall`. The operation is only valid for
an existing thumbnail so the `hasThumbnail` remains unchanged.
This patch also clarifies that the "flipped" classes are exclusively set
by using an if / else chain instead of reconsidering all inputs for each
condition.
Bug: T190831
Change-Id: I4062ec7068dcadecbdbc4791447ea2ed1ce2a1de
