Add test cases for hIST chunk

contrib/testpngs/ch1n3p04.png

diff --git a/contrib/testpngs/ch1n3p04.png b/contrib/testpngs/ch1n3p04.png
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index 0000000..17cd12d
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+++ b/contrib/testpngs/ch1n3p04.png
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contrib/testpngs/ch2n3p08.png

diff --git a/contrib/testpngs/ch2n3p08.png b/contrib/testpngs/ch2n3p08.png
new file mode 100644
index 0000000..25c1798
--- /dev/null
+++ b/contrib/testpngs/ch2n3p08.png
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