PSD reading: don't reject padded thumbnails (#3677)

When reading PSD file, if reading the thumbnail doesn't pass basic
sanity checks, we consider that a hard error reading the file.  One
check that we do is to make sure that our computation of the byte
width of the uncompressed thumbnail (`bpp/8 * width`) matches what the
file says the `widthbytes` is.

BUT... it seems that it might round up to an even (or maybe 4-multiple?)
of columns, presumably for scanline alignment reasons. And so we were
considering whole PDF files unreadable if they fell into this case.

This PR primarily loosens the test to allow some padding up to a
multiple of 4.

Fixes #3658

Author: lgritz

src/psd.imageio/psdinput.cpp

@@ -1282,7 +1282,7 @@ PSDInput::load_resource_thumbnail(uint32_t length, bool isBGR)
             bpp);
         return false;
     }
-    if ((bpp / 8) * width != widthbytes) {
+    if ((bpp / 8) * width > widthbytes || (bpp / 8) * width + 3 < widthbytes) {
         errorfmt("Corrupt thumbnail: {}w * {}bpp does not match {} width bytes",
                  width, bpp, widthbytes);
         return false;
@@ -1311,9 +1311,10 @@ PSDInput::load_resource_thumbnail(uint32_t length, bool isBGR)
     m_thumbnail.read(0, 0, true);
 
     // Set these attributes for the merged composite only (subimage 0)
-    composite_attribute("thumbnail_width", (int)width);
-    composite_attribute("thumbnail_height", (int)height);
-    composite_attribute("thumbnail_nchannels", 3);
+    composite_attribute("thumbnail_width", (int)m_thumbnail.spec().width);
+    composite_attribute("thumbnail_height", (int)m_thumbnail.spec().height);
+    composite_attribute("thumbnail_nchannels",
+                        (int)m_thumbnail.spec().nchannels);
     if (isBGR)
         m_thumbnail = ImageBufAlgo::channels(m_thumbnail, 3, { 2, 1, 0 });
     return true;