Closes https://github.com/ppy/osu/issues/33748.
I (and tests) can't find any regressions from this. One would hope we
aren't relying on fall-through mouse down anywhere beneath buttons..
Keep masking back in `Content`, since the scaling animation is happening
on `Content` instead of `this`. This doesn't regress the intended
behaviour in this PR (which is to just to make the button class itself
sheared instead of its content).
Saves having this defined in 20+ places. If we ever make any changes to
shear, it's 100% going to need to be applied to every usage (there will
never be a case of multiple different shears in the game).
Also fixes a mismatching definition in `ShearedNub`.
Also includes a test case in `TestSceneShearedButton`s to ensure the buttons' shear factors don't change on different heights (I've encountered issues with that previously).
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.