Plot auto-bounds API improvement (part 1/2): clean-up (#3587)

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`

crates/emath/src/vec2b.rs

@@ -1,5 +1,5 @@
 /// Two bools, one for each axis (X and Y).
-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct Vec2b {
     pub x: bool,
@@ -58,3 +58,15 @@ impl std::ops::IndexMut<usize> for Vec2b {
         }
     }
 }
+
+impl std::ops::Not for Vec2b {
+    type Output = Self;
+
+    #[inline]
+    fn not(self) -> Self::Output {
+        Self {
+            x: !self.x,
+            y: !self.y,
+        }
+    }
+}