Replace pdfimages -list call to poppler with PyPDF test for image

The immediate reason for doing this is that (newer?) versions of parse()
seem to choke on the parse string. It appears to trigger exponential
behavior in the underlying regex. In any case, replacing subprocesses
with native Python is usually better.

src/ocrpage.py

@@ -145,22 +145,21 @@ def pdf_get_pageinfo(infile, page, width_pt, height_pt):
     else:
         pageinfo['has_text'] = False
 
-    # pdfimages: get image dimensions
-    p_pdfimages = Popen(['pdfimages', '-list', '-f', str(page), '-l',
-                        str(page), str(infile)], close_fds=True, stdout=PIPE,
-                        stderr=PIPE, universal_newlines=True)
-    pdfimages, _ = p_pdfimages.communicate()
-    for n, line in enumerate(pdfimages.splitlines()):
-        if n <= 1:
-            continue  # Skip first two lines
+    pdf = pypdf.PdfFileReader(infile)
+    page = pdf.pages[page - 1]
 
-        r = parse('{page:1d} {num:1d} {imtype:>} {width:1d} {height:1d} ' +
-                  '{color:>} {comp:1d} {bpc:1d} {enc:>} {interp:>} ' +
-                  '{pdfobject:1d} {pdfid:1d} {bad_dpi_w:1d} {bad_dpi_h:1d} ' +
-                  '{size:>} {ratio:>}', line)
-        image = r.named
-        # pdfimages calculates DPI as of 0.26.0, but adds +1 to dpi_h
-        # apparent bug, so calculate explicitly
+    if not '/XObject' in page['/Resources']:
+        # Missing /XObject means no images or possibly corrupt PDF
+        return pageinfo
+
+    for xobj in page['/Resources']['/XObject']:
+        # PyPDF2 returns the keys as an iterator
+        pdfimage = page['/Resources']['/XObject'][xobj]
+        if pdfimage['/Subtype'] != '/Image':
+            continue
+        image = {}
+        image['width'] = pdfimage['/Width']
+        image['height'] = pdfimage['/Height']
         image['dpi_w'] = image['width'] / pageinfo['width_inches']
         image['dpi_h'] = image['height'] / pageinfo['height_inches']
         image['dpi'] = (image['dpi_w'] * image['dpi_h']) ** 0.5