Implement draw.py to draw rounded rectangles

lib/xmet/draw.py

@@ -0,0 +1,29 @@
+import math
+import cairo
+
+def draw_rounded_rect(cr: cairo.Context,
+                      x: float,
+                      y: float,
+                      width: float,
+                      height: float,
+                      radius: float):
+    rect_width  = width  - 2 * radius
+    rect_height = height - 2 * radius
+
+    # Upper right
+    cr.arc(x + radius + rect_width, y + radius, radius,
+           -math.pi / 2, 0)
+
+    # Lower right
+    cr.arc(x + radius + rect_width, y + radius + rect_height, radius,
+           0, math.pi / 2)
+
+    # Lower left
+    cr.arc(x + radius, y + radius + rect_height, radius,
+           math.pi / 2, math.pi)
+
+    # Upper right
+    cr.arc(x + radius, y + radius, radius,
+           math.pi, math.pi * 1.5)
+
+    cr.line_to(x + radius + rect_width, y)