X11: Fix get_current_monitor (#515)

* X11: Fix get_current_monitor

Fixes #64

* impl Debug for MonitorId on all platforms

src/platform/linux/x11/util/geometry.rs

@@ -1,5 +1,40 @@
+use std::cmp;
+
 use super::*;
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Rect {
+    left: i64,
+    right: i64,
+    top: i64,
+    bottom: i64,
+}
+
+impl Rect {
+    pub fn new((x, y): (i32, i32), (width, height): (u32, u32)) -> Self {
+        let (x, y) = (x as i64, y as i64);
+        let (width, height) = (width as i64, height as i64);
+        Rect {
+            left: x,
+            right: x + width,
+            top: y,
+            bottom: y + height,
+        }
+    }
+
+    pub fn get_overlapping_area(&self, other: &Self) -> i64 {
+        let x_overlap = cmp::max(
+            0,
+            cmp::min(self.right, other.right) - cmp::max(self.left, other.left),
+        );
+        let y_overlap = cmp::max(
+            0,
+            cmp::min(self.bottom, other.bottom) - cmp::max(self.top, other.top),
+        );
+        x_overlap * y_overlap
+    }
+}
+
 #[derive(Debug)]
 pub struct TranslatedCoords {
     pub x_rel_root: c_int,