optimize: fix all JBIG2 images binned on last page

During some past refactor it appears we now end up treating
all JBIG2 images as if they appeared on the last page in the
file. This bug had no visual side ffects but probably led to
suboptimal JBIG2 encoding.

src/ocrmypdf/optimize.py

@@ -149,11 +149,22 @@ def extract_image_generic(*, pike, root, log, image, xref, options):
 def extract_images(pike, root, log, options, extract_fn):
     """Extract image using extract_fn
 
-    extract_fn decides whether the image is interesting in this case
+    Enumerate images on each page, lookup their xref/ID number in the PDF.
+    Exclude images that are soft masks (i.e. alpha transparency related).
+    Record the page number on which an image is first used, since images may be
+    used on multiple pages (or multiple times on the same page).
+
+    Current we do not check Form XObjects or other objects that may contain
+    images, and we don't evaluate alternate images or thumbnails.
+
+    extract_fn must decide if wants to extract the image in this context. If
+    it does a tuple should be returned: (xref, ext) where .ext is the file
+    extension. extract_fn must also extract the file it finds interesting.
     """
 
     include_xrefs = set()
     exclude_xrefs = set()
+    pageno_for_xref = {}
     errors = 0
     for pageno, page in enumerate(pike.pages):
         try:
@@ -169,6 +180,8 @@ def extract_images(pike, root, log, options, extract_fn):
                 smask_xref = image.SMask.objgen[0]
                 exclude_xrefs.add(smask_xref)
             include_xrefs.add(xref)
+            if xref not in pageno_for_xref:
+                pageno_for_xref[xref] = pageno
 
     working_xrefs = include_xrefs - exclude_xrefs
     for xref in working_xrefs:
@@ -178,13 +191,12 @@ def extract_images(pike, root, log, options, 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))
+            log.debug("Image xref %s, error %s", xref, repr(e))
             errors += 1
         else:
             if result:
                 _, ext = result
-                yield pageno, xref, ext
+                yield pageno_for_xref[xref], xref, ext
 
 
 def extract_images_generic(pike, root, log, options):
@@ -198,7 +210,7 @@ def extract_images_generic(pike, root, log, options):
             pngs.append(xref)
         elif ext == '.jpg':
             jpegs.append(xref)
-    log.debug("Optimizable images: " "JPEGs: %s PNGs: %s", len(jpegs), len(pngs))
+    log.debug("Optimizable images: JPEGs: %s PNGs: %s", len(jpegs), len(pngs))
     return jpegs, pngs
 
 
@@ -216,7 +228,7 @@ def extract_images_jbig2(pike, root, log, options):
     jbig2_groups = {
         group: xrefs for group, xrefs in jbig2_groups.items() if len(xrefs) > 0
     }
-    log.debug("Optimizable images: " "JBIG2 groups: %s", (len(jbig2_groups),))
+    log.debug("Optimizable images: JBIG2 groups: %s", (len(jbig2_groups),))
     return jbig2_groups