Fix review findings in the shared wipe tower estimate

A raft is not a reason to reserve a tower. Print::apply runs
normalize_fdm_2, which clears enable_prime_tower for a plate that purges
one filament unless smooth timelapse or wrapping detection is on, so a
single-filament plate with a raft prints no tower at all and the estimate
was reserving bed area for one. Drop the input; need_wipe_tower is now
exactly the two exceptions normalize_fdm_2 honours, named there so the
next reason added has to be checked against it.

The GUI preview and the validation containment check each re-derived
"is a tower printed here" from the filament count instead of reading the
estimate, so both missed the towers printed with no tool change to purge
for. They now take the answer from the footprint, which is the drift this
shared estimate exists to remove. A tower that is not printed estimates to
zero, so its hull is degenerate and every check on it passes trivially -
the containment check needs no gate of its own.

WipeTowerData::width was written only by the pre-generation estimate and
left at zero for the whole post-generation life of the Print, while its
neighbour depth held the real value. Set it from the generator in both
branches.

The plate's height scan transformed every model part's full mesh per
instance on each scene reload, discarding all but the z extent. The
cached convex hull has the same z extent.

A plate loaded from a sliced .gcode.3mf holds no objects and its filaments
live in slice_filaments_info; the config-taking get_extruders overload
returned an empty list for it, which sized the tower for a placeholder two
filaments. It now answers the way the wx overload does, without reaching
the plater.

Also drop estimate_wipe_tower_size, which has no callers.

src/libslic3r/GCode/WipeTowerEstimate.cpp

@@ -11,7 +11,7 @@
 
 namespace Slic3r {
 
-WipeTowerFootprint estimate_wipe_tower_footprint(const ConfigBase &config, size_t filaments_cnt, double layer_height, double max_object_height, bool any_raft)
+WipeTowerFootprint estimate_wipe_tower_footprint(const ConfigBase &config, size_t filaments_cnt, double layer_height, double max_object_height)
 {
     WipeTowerFootprint footprint;
     footprint.height = max_object_height;
@@ -56,8 +56,9 @@ WipeTowerFootprint estimate_wipe_tower_footprint(const ConfigBase &config, size_
     const bool   dual_nozzle      = nozzle_opt != nullptr && nozzle_opt->values.size() == 2;
     const bool   rib_wall         = opt_enum("wipe_tower_wall_type", int(WipeTowerWallType::wtwRectangle)) == int(WipeTowerWallType::wtwRib);
     const bool   smooth_timelapse = opt_enum("timelapse_type", int(TimelapseType::tlTraditional)) == int(TimelapseType::tlSmooth);
-    // Reasons a tower is printed with no tool change to purge for.
-    const bool   need_wipe_tower  = smooth_timelapse || opt_bool("enable_wrapping_detection") || any_raft;
+    // Reasons a tower is printed with no tool change to purge for: the ones that stop
+    // normalize_fdm_2 clearing enable_prime_tower. Its mixed-filament case is not modelled.
+    const bool   need_wipe_tower  = smooth_timelapse || opt_bool("enable_wrapping_detection");
 
     // No tool change, nothing to purge; smooth timelapse still primes once.
     size_t purge_count = 0;
@@ -80,7 +81,9 @@ WipeTowerFootprint estimate_wipe_tower_footprint(const ConfigBase &config, size_
 
     // Both wall types decide this together: over-reserving only wastes bed area, but
     // reporting no tower for one that is built collapses the validation hull to a point.
-    if (volume < EPSILON && !need_wipe_tower)
+    // A tool change is a reason on its own: the generator floors the tower whatever the
+    // purge volumes resolve to.
+    if (volume < EPSILON && filaments_cnt < 2 && !need_wipe_tower)
         return footprint;
 
     const double min_depth = WipeTower::get_limit_depth_by_height(float(max_object_height));