Add Image::rotate and Mesh::rotate (#1371)

Co-authored-by: Hunter Morgan <hmorgan@bellflight.com>

epaint/src/mesh.rs

@@ -254,6 +254,15 @@ impl Mesh {
             v.pos += delta;
         }
     }
+
+    /// Rotate by some angle about an origin, in-place.
+    ///
+    /// Origin is a position in screen space.
+    pub fn rotate(&mut self, rot: Rot2, origin: Pos2) {
+        for v in &mut self.vertices {
+            v.pos = origin + rot * (v.pos - origin);
+        }
+    }
 }
 
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