slic3r: address sanitizer cleanup in TreeSupport::generate_contact_points

In generate_contact_points, the rotated_dims don't actually generate a
Point, but generate an abstract expression that holds a reference to
bounding_box_size.  Unfortunately, this goes wrong because bounding_box_size
ages out instantly; GCC's Address Sanitizer complains that this is a
"stack-use-after-scope" issue.

Interestingly, the Eigen documentation says that this is a known problem
with using C++11 "auto" to generate matrices (which a Point is)!  It is
buried here: https://eigen.tuxfamily.org/dox/TopicPitfalls.html

This is probably an extremely theoretical correctness improvement, and also
an extremely theoretical performance improvement.  But it is an improvement
nonetheless, and, more importantly, it enables me to continue to run the
slicer with Address Sanitizer turned on to find other potential issues.

src/libslic3r/TreeSupport.cpp

@@ -3330,7 +3330,7 @@ void TreeSupport::generate_contact_points(std::vector<std::vector<TreeSupport::N
     const auto center = bounding_box_middle(bounding_box);
     const auto sin_angle = std::sin(rotate_angle);
     const auto cos_angle = std::cos(rotate_angle);
-    const auto rotated_dims = Point(
+    const Point rotated_dims = Point(
         bounding_box_size(0) * cos_angle + bounding_box_size(1) * sin_angle,
         bounding_box_size(0) * sin_angle + bounding_box_size(1) * cos_angle) / 2;