Store original and PDF stream images in different cache slots

We used to replace the original image by the PDF stream image in cache after
the first PDF rendering. It’s broken because the PDF stream image is not a
valid image, and can’t be opened by Pillow if it has to be rendered twice.

Replacing the original image with the final one was probably supposed to save
some time and/or some memory, but my quick tests show no performance problem
with this commit.

Fix #1942, fix #2228.

weasyprint/images.py

@@ -114,11 +114,11 @@ def draw(self, stream, concrete_width, concrete_height, image_rendering):
             concrete_width, 0, 0, -concrete_height, 0, concrete_height)
         stream.draw_x_object(image_name)
 
-    def cache_image_data(self, data, filename=None, alpha=False):
+    def cache_image_data(self, data, filename=None, slot='source'):
         if filename:
             return LazyLocalImage(filename)
         else:
-            key = f'{self.id}{int(alpha)}{self._dpi or ""}'
+            key = f'{self.id}-{slot}-{self._dpi or ""}'
             return LazyImage(self._cache, key, data)
 
     def get_x_object(self, interpolate, dpi_ratio):
@@ -183,7 +183,7 @@ def get_x_object(self, interpolate, dpi_ratio):
             png_data = self._get_png_data(pillow_image)
             # Save alpha channel as mask
             alpha_data = self._get_png_data(alpha)
-            stream = self.cache_image_data(alpha_data, alpha=True)
+            stream = self.cache_image_data(alpha_data, slot='streamalpha')
             extra['SMask'] = pydyf.Stream([stream], extra={
                 'Filter': '/FlateDecode',
                 'Type': '/XObject',
@@ -202,7 +202,7 @@ def get_x_object(self, interpolate, dpi_ratio):
             png_data = self._get_png_data(
                 Image.open(io.BytesIO(self.image_data.data)))
 
-        return pydyf.Stream([self.cache_image_data(png_data)], extra)
+        return pydyf.Stream([self.cache_image_data(png_data, slot='stream')], extra)
 
     @staticmethod
     def _get_png_data(pillow_image):