(Reland) Fix extraction of colour components in CPDF_DIBSource::DownSampleScanline32Bit

Previously, if |m_bpc| was < 8 (e.g. 4), this function may still try to
access the source components as if |m_bpc| == 8. Even when it fell into
the codepath that tried to do the right thing in this case, it was
wrong.

BUG=554151
R=tsepez@chromium.org, thestig@chromium.org

Committed: https://pdfium.googlesource.com/pdfium/+/9b99615806e358fdb396d1cb162ee2e69c2a20ec

Review URL: https://codereview.chromium.org/1433423002 .

2 files changed, 129 insertions, 0 deletions

diff --git a/testing/resources/bug_554151.in b/testing/resources/bug_554151.in
new file mode 100644
index 0000000000..3087159f51
--- /dev/null
+++ b/testing/resources/bug_554151.in
@@ -0,0 +1,59 @@
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+
+{{object 1 0}}
+<<
+  /Pages 2 0 R
+>>
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+<<
+  /Kids [ 3 0 R ]
+>>
+endobj
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+<<
+  /Contents 6 0 R
+  /Resources 5 0 R
+>>
+endobj
+
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+<<>>
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+612 0.0 0.0 104000 0 0 cm
+/XBad Do
+endstream
+endobj
+
+{{object 5 0}}
+<<
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+    /XBad 7 0 R
+  >>
+>>
+endobj
+
+{{object 7 0}}
+<<
+  /Type /XObject
+  /Subtype /Image
+  /Width 612
+  /Height 104000
+  /ColorSpace /DeviceRGB
+  /BitsPerComponent 4
+  /Length 0
+  /Decode [1.0]
+>>
+stream
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+endobj
+
+{{xref}}
+trailer <<
+  /Root 1 0 R
+  /Size 8
+>>
+{{startxref}}
+%%EOF
diff --git a/testing/resources/bug_554151.pdf b/testing/resources/bug_554151.pdf
new file mode 100644
index 0000000000..fa770a7b37
--- /dev/null
+++ b/testing/resources/bug_554151.pdf
@@ -0,0 +1,70 @@
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+<<
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+  /Resources 5 0 R
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+/XBad Do
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+  >>
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+
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+  /ColorSpace /DeviceRGB
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+  /Decode [1.0]
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