Noticed while testing on iOS. Previously, the interpolation was being
done in input handling but using the update thread clock, leading to
incorrect application.
The border would previously get brighter after click, but then dim
instantly when the flash layer has fully faded out. The underlying issue
there is https://github.com/ppy/osu-framework/issues/5191, but
`ShearedButton` was placing the flashing layer incorrectly anyway, as
the intent was that it should also apply to the border.