DragValue speed skips legit values

I set the speed of a `DragValue` to 0.1 and for some reason it always skips `0.4` and `0.9`.

This is even reproducible on the egui.rs version, simply hold shift (to slow the speed down)

https://github.com/user-attachments/assets/0132d5aa-ac87-4a99-9659-bf3c3c56c6de

Start by reproducing the issue in the DragValue example on egui.rs, using a speed of 0.1 and holding Shift. Trace the DragValue input and value-step handling to find why 0.4 and 0.9 are skipped. Done means legitimate decimal values can be reached without skips and the reproduction no longer occurs.

Written by the indexing model from the issue text.