Part 1 of 2 of adding a better API for egui_plot's auto-bounds feature.
In this PR:
- change the `Plot` builder struct field to `default_auto_bounds` (was
`auto_bounds`)
- change the `Plot` state field to `auto_bounds` (was `bounds_modified`)
- minor improvements to `Vec2b`
Thanks to `impl From<bool> for Vec2b` one can now shorten some builder
calls, like:
Previous:
```rust
egui::ScrollArea::vertical()
.auto_shrink([false; 2])
```
New:
```rust
egui::ScrollArea::vertical()
.auto_shrink(false)
```
Emil Ernerfeldt
Antoine Beyeler