WIP: iconGrid: Remove _paintVisible opacity tracking

Daniel van Vugt requested to merge vanvugt/gnome-shell:unspring-spike into master

Because it was causing a number of frames (<= 24) of the spring animation to incur relayouts. So many that worst-case render times grew by ~100ms.

Its purpose was to fix #1502 (closed), but we can do that more efficiently on the one frame after the animation finishes.

Closes: #2065 (closed)
Related: #349 (closed)

Edited by Jeff Fortin Tam

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