optimize: Don't reinsert 1bpp images

There seems to be version to version inconsistencies between
Leptonica's photometric interpretation of 1bpp images, in
particular commit a0692307 introduces a change to force transcoding
in this situation.

However, I never entirely got to the bottom of where the problem
is, and in any event 1bpp images are probably better optimized
by JBIG2 than pngquant, so we're going to stop running them through
pngquant.

src/ocrmypdf/optimize.py

@@ -392,6 +392,13 @@ def transcode_pngs(pike, images, image_name_fn, root, log, options):
             )
             continue
 
+        if compdata.bps == 1:
+            # Discard 1bpp images due to issues with preserving the photometric
+            # interpretation. In particular Leptonica changed behavior in
+            # version 1.77.0, such that it transcodes 1bpp.
+            log.debug(f"discarded optimized image {xref} because it was 1bpp")
+            continue
+
         # When a PNG is inserted into a PDF, we more or less copy the IDAT section from
         # the PDF and transfer the rest of the PNG headers to PDF image metadata.
         # One thing we have to do is tell the PDF reader whether a predictor was used
@@ -440,12 +447,6 @@ def transcode_pngs(pike, images, image_name_fn, root, log, options):
         else:
             # ncolors == 0 means we are using a colorspace without a palette
             if compdata.spp == 1:
-                if compdata.bps == 1:
-                    # PDF interprets binary-1 as black in 1bpp, but PNG sets
-                    # black to 0 for 1bpp. Use Decode to ensure color is
-                    # correct.
-                    log.debug("Inverting photometry")
-                    im_obj.Decode = [1, 0]
                 cs = Name.DeviceGray
             elif compdata.spp == 3:
                 cs = Name.DeviceRGB