PNG palette: parse PDF string from leptonica instead

Seems better to accept whatever leptonica rather than make detailed
assumptions about how it encodes the palette.

Experimented with setting FlateDecode on the palette but it seems to
expand it.

src/ocrmypdf/leptonica.py

@@ -27,7 +27,6 @@ import logging
 import warnings
 from tempfile import TemporaryFile
 from ctypes.util import find_library
-from binascii import unhexlify
 from .lib._leptonica import ffi
 from functools import lru_cache
 from enum import Enum
@@ -518,12 +517,11 @@ class CompressedData:
             return getattr(self._compdata, name)
         raise AttributeError(name)
 
-    def get_palette(self):
-        buf = ffi.buffer(self._compdata.cmapdatahex, 
-                         self._compdata.ncolors * 7 + 3)
-        hex_str = bytes(buf).decode('ascii')
-        hex_str = hex_str[1:-1].replace(' ', '')
-        return unhexlify(hex_str)
+    def get_palette_pdf_string(self):
+        "Returns palette pre-formatted for use in PDF"
+        buflen = len('< ') + len(' rrggbb') * self._compdata.ncolors + len('>')
+        buf = ffi.buffer(self._compdata.cmapdatahex, buflen)
+        return bytes(buf)
 
     @staticmethod
     def _destroy(compdata):

src/ocrmypdf/pipeline.py

@@ -1176,10 +1176,12 @@ def optimize_pdf(
         im_obj.Height = compdata.h
 
         if compdata.ncolors > 0:
-            palette_stream = pikepdf.Stream(pike, compdata.get_palette())
+            palette_pdf_string = compdata.get_palette_pdf_string()
+            log.info(palette_pdf_string)
+            palette_data = pikepdf.Object.parse(palette_pdf_string)
+            palette_stream = pikepdf.Stream(pike, bytes(palette_data))
             palette = [pikepdf.Name('/Indexed'), pikepdf.Name('/DeviceRGB'),
                        compdata.ncolors - 1, palette_stream]
-            #cs = pike.make_indirect(palette)
             cs = palette
         else:
             if compdata.spp == 1:
@@ -1192,13 +1194,13 @@ def optimize_pdf(
 
         im_obj.write(compdata.read(), pikepdf.Name('/FlateDecode'), predictor)
 
-        
-
+    # Not object_stream_mode + preserve_pdfa generates noncompliant PDFs
     pike.save(output_file, preserve_pdfa=True)
+
     input_size = Path(input_file).stat().st_size
     output_size = Path(output_file).stat().st_size
     improvement = input_size / output_size
-    log.info("Optimize reduced size by {:.1f}".format(improvement))
+    log.info("Optimize reduced size by {:.3f}".format(improvement))
 
 
 def merge_sidecars(