Wipe tower postprocessing, wipe tower block on 3D plate improved.

- it renders red with one egde as indeterminate, the front edge is where the wipe tower will start
- changing width changes depth of the block (as requested)
- the block shows the brim of the wipe tower
- after slicing, the block is rendered in usual dark green and takes the exact shape of the tower (also with brim)
- moving or rotationg the block after slicing does not invalidate the wipe tower (and hence the exact block dimensions are preserved)
- changing anything that invalidates the wipe tower reverts the block back to the "indeterminate" shape
- the block is not shown after slicing, if the wipe tower is not actually generated (printing single color object with the wipe tower enabled)

This required changes in the wipe tower generator, which now generates the tower
at origin with no rotation. Resulting gcode is postprocessed and transformed during
gcode export. This means the wipe tower needs not be invalidated when it is moved or rotated.

xs/src/libslic3r/Print.hpp

@@ -273,6 +273,7 @@ public:
     
     void add_model_object(ModelObject* model_object, int idx = -1);
     bool apply_config(DynamicPrintConfig config);
+    float get_wipe_tower_depth() const { return m_wipe_tower_depth; }
     bool has_infinite_skirt() const;
     bool has_skirt() const;
     // Returns an empty string if valid, otherwise returns an error message.
@@ -326,6 +327,9 @@ private:
     bool invalidate_state_by_config_options(const std::vector<t_config_option_key> &opt_keys);
     PrintRegionConfig _region_config_from_model_volume(const ModelVolume &volume);
 
+    // Depth of the wipe tower to pass to GLCanvas3D for exact bounding box:
+    float m_wipe_tower_depth = 0.f;
+
     // Has the calculation been canceled?
     tbb::atomic<bool>   m_canceled;
 };