ScrollArea::show_rows() scrolling jitters with very large total_rows

**Describe the bug**

I have a UI with about 16 million rows (but this is on the low end of what I want to support). I'm using `ScrollArea::show_rows()` to keep a sane viewport size.

Up to about 2 million rows, scrolling with the trackpad or by dragging the background with a mouse is relatively smooth. Above 2 million total rows, scrolling has some obvious jitter. Above 100 million total rows, the scrolling gets very broken.

It looks like an issue with `f32` precision, but I haven't been able to track it down yet.

**To Reproduce**
Steps to reproduce the behavior:
1. Patch the demo lib to increase the `num_rows` to `16_000_000`: https://github.com/emilk/egui/blob/ccbddcfe951e01c55efd0ed19f2f2ab5edfad5d9/egui_demo_lib/src/apps/demo/scrolling.rs#L85
2. Run the demo and go to the "Scolling" demo and then the "Scroll a lot of lines" tab.
3. Scroll with the trackpad or by dragging the background with the mouse.
4. You'll see it does not scroll smoothly.
5. Increase `num_rows` to `160_000_000` and try the same test.
6. Scrolling is almost completely unusable with this many rows.

**Expected behavior**

Always scroll smoothly regardless of the total number of rows.

**Desktop (please complete the following information):**
- OS: Windows 11, macOS 12.2.1

Start with egui_demo_lib/src/apps/demo/scrolling.rs at the num_rows setting and run the “Scroll a lot of lines” demo with 16,000,000 and 160,000,000 rows. Trace how ScrollArea::show_rows handles scrolling and large coordinates, then verify that trackpad scrolling and mouse dragging remain smooth at those sizes.

Written by the indexing model from the issue text.