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/GCode/WipeTowerPrusaMM.hpp

@@ -102,6 +102,8 @@ public:
 	// Iterates through prepared m_plan, generates ToolChangeResults and appends them to "result"
 	void generate(std::vector<std::vector<WipeTower::ToolChangeResult>> &result);
 
+    float get_depth() const { return m_wipe_tower_depth; }
+
 
 
 	// Switch to a next layer.
@@ -189,6 +191,7 @@ private:
 	float  m_wipe_tower_width; 			// Width of the wipe tower.
 	float  m_wipe_tower_depth 	= 0.f; 	// Depth of the wipe tower
 	float  m_wipe_tower_rotation_angle = 0.f; // Wipe tower rotation angle in degrees (with respect to x axis)
+    float  m_internal_rotation  = 0.f;
 	float  m_y_shift			= 0.f;  // y shift passed to writer
 	float  m_z_pos 				= 0.f;  // Current Z position.
 	float  m_layer_height 		= 0.f; 	// Current layer height.