Add anchors to windows and areas (#310)

This is so that you can put a window in e.g. the top right corner
or the center of the screen.

emath/src/align.rs

@@ -52,6 +52,15 @@ impl Align {
             Self::Max => 1.0,
         }
     }
+
+    /// Convert `Min => -1.0`, `Center => 0.0` or `Max => 1.0`.
+    pub fn to_sign(&self) -> f32 {
+        match self {
+            Self::Min => -1.0,
+            Self::Center => 0.0,
+            Self::Max => 1.0,
+        }
+    }
 }
 
 impl Default for Align {
@@ -88,6 +97,11 @@ impl Align2 {
         self.0[1]
     }
 
+    /// -1, 0, or +1 for each axis
+    pub fn to_sign(&self) -> Vec2 {
+        vec2(self.x().to_sign(), self.y().to_sign())
+    }
+
     /// Used e.g. to anchor a piece of text to a part of the rectangle.
     /// Give a position within the rect, specified by the aligns
     pub fn anchor_rect(self, rect: Rect) -> Rect {
@@ -119,6 +133,21 @@ impl Align2 {
 
         Rect::from_min_size(Pos2::new(x, y), size)
     }
+
+    pub fn pos_in_rect(self, frame: &Rect) -> Pos2 {
+        let x = match self.x() {
+            Align::Min => frame.left(),
+            Align::Center => frame.center().x,
+            Align::Max => frame.right(),
+        };
+        let y = match self.y() {
+            Align::Min => frame.top(),
+            Align::Center => frame.center().y,
+            Align::Max => frame.bottom(),
+        };
+
+        pos2(x, y)
+    }
 }
 
 pub fn center_size_in_rect(size: Vec2, frame: Rect) -> Rect {