fix(slider): prevent slider thumb from getting cropped

[alexshevch]

Slider thumb may sometimes get cropped by a scale transformation.
Removing backface visibility ensures that the thumb is painted correctly.

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[alexshevch]

I signed it.

[mmalerba]

are you sure you used the same account to sign it as you did to create the commits? it should automatically update if its signed properly

[alexshevch]

Thank you for pointing it out, @mmalerba. Google pulled up a different email when I was signing a CLA. I signed it!

[googlebot]

CLAs look good, thanks!

[mmalerba]

great, thanks!

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