optimize: Exclude soft masks (SMasks) from optimization

Soft masks are only allowed to be of colorspace DeviceGray so we
shouldn't use pngquant on them. For now, avoid this exceptional
case by excluded soft masks from optimization.

src/ocrmypdf/optimize.py

@@ -149,33 +149,40 @@ def extract_images(pike, root, log, options, extract_fn):
     extract_fn decides where the image is interesting in this case
     """
 
-    changed_xrefs = set()
+    include_xrefs = set()
+    exclude_xrefs = set()
     errors = 0
     for pageno, page in enumerate(pike.pages):
         try:
             xobjs = page.Resources.XObject
         except AttributeError:
             continue
-        for imname, image in dict(xobjs).items():
+        for _imname, image in dict(xobjs).items():
             if image.objgen[1] != 0:
                 continue  # Ignore images in an incremental PDF
             xref = image.objgen[0]
-            if xref in changed_xrefs:
-                continue  # Don't improve same image twice
-            try:
-                result = extract_fn(
-                    pike=pike, root=root, log=log, image=image,
-                    xref=xref, options=options
-                )
-            except Exception as e:
-                log.debug("Image %s xref %s", imname, xref)
-                log.debug(repr(e))
-                errors += 1
-            else:
-                if result:
-                    changed_xrefs.add(xref)
-                    _, ext = result
-                    yield pageno, xref, ext
+            if hasattr(image, 'SMask'):
+                # Ignore soft masks
+                smask_xref = image.SMask.objgen[0]
+                exclude_xrefs.add(smask_xref)
+            include_xrefs.add(xref)
+
+    working_xrefs = include_xrefs - exclude_xrefs
+    for xref in working_xrefs:
+        image = pike.get_object((xref, 0))
+        try:
+            result = extract_fn(
+                pike=pike, root=root, log=log, image=image,
+                xref=xref, options=options
+            )
+        except Exception as e:
+            log.debug("Image xref %s", xref)
+            log.debug(repr(e))
+            errors += 1
+        else:
+            if result:
+                _, ext = result
+                yield pageno, xref, ext
 
 
 def extract_images_generic(pike, root, log, options):