Defer freeing histogram data to remapping

So that an image can be quantized multiple times

Author: kornelski

src/hist.rs

@@ -132,7 +132,6 @@ impl Histogram {
         self.reserve(estimated_colors);
 
         self.add_pixel_rows(&image.px, image.importance_map.as_deref(), posterize_bits)?;
-        image.free_histogram_inputs();
 
         Ok(())
     }

src/quant.rs

@@ -87,6 +87,8 @@ impl QuantizationResult {
             return Err(Error::Aborted);
         }
 
+        image.free_histogram_inputs();
+
         let mut palette = self.palette.clone();
         let mut remapped = Box::new(Remapped {
             int_palette: Palette { count: 0, entries: [RGBA::default(); MAX_COLORS] },