Fix vertical Slider regression(?)

[hinto-janai]

Regression(?)

In egui 0.23.0, this commit and line exactly, the behavior of vertical Slider's has been swapped.

I'm not sure if this was intentional (nothing in the CHANGELOG.md).

The base used to be the bottom, but it is now the top.

This PR reverts the code calculating the start position and uses .into() to convert to the new Rangef.

egui 0.23.0 Behavior

Horizontal sliders increment left <-> right, vertical sliders increment top <-> bottom:

 0 <-------> 100

       0
       ^
       |
       v
      100

trailing_fill behavior is incorrect here.

current.mp4

Fixed(?) Behavior

Horizontal sliders increment left <-> right (same), vertical sliders increment bottom <-> top (old behavior):

 0 <-------> 100

      100
       ^
       |
       v
       0

fix.mp4

[YgorSouza]

There is already an issue and PR for this #3422 #3424.

[hinto-janai]

Ah, I didn't see #3424, it's definitely a better fix - I'll leave this PR up for visibility until 3424 is merged.

[emilk]

• Fixed in Fix upside down slider in the vertical orientation #3424 instead