rcfilters: Stop the "Live updates" button animation from affecting text rendering

rcfilters: Stop the "Live updates" button animation from affecting text rendering

Add 'z-index: 0' to introduce a stacking context. Stacking contexts
fix everything. Apparently.

In this case, this prevents browsers (tested with Chrome and Firefox)
from using grayscale text antialiasing instead of the usual subpixel
text antialiasing for the rest of the page; it is only used for this
button, which is acceptable.

(Firefox has some problems rendering the animation, but they already
existed before this patch.)

Bug: T181338
Change-Id: I059f819eb27b0f22fc2d09a2c61e5e65af878ed4