Further optimize img!

/encoder_lvl_vs_perf.png -- 7.22kb -> 6.18kb (14.4%)
/webgl/texture/preview.png -- 432.41kb ->  317.87kb (26.488%)
/contrib/single_file_transcoder/examples/testcard.png -- 10.55kb ->  7.90kb (25.1%)
/contrib/single_file_transcoder/examples/testcard-rgba.png -- 14.3kb ->  11.52kb (19.44%)
/webgl/gltf/preview.png -- 490.65kb ->  458kb (6.652%)
/contrib/previewers/win/preview.png -- 652.43kb ->  624.31 (4.31%)
Total:  1,609.39‬kb  -> 1425.78kb  (11.4%)
diff --git a/contrib/previewers/win/preview.png b/contrib/previewers/win/preview.png
index 9513b8b..bb59492 100644
--- a/contrib/previewers/win/preview.png
+++ b/contrib/previewers/win/preview.png
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diff --git a/contrib/single_file_transcoder/examples/testcard-rgba.png b/contrib/single_file_transcoder/examples/testcard-rgba.png
index 7f601a1..c4200e8 100644
--- a/contrib/single_file_transcoder/examples/testcard-rgba.png
+++ b/contrib/single_file_transcoder/examples/testcard-rgba.png
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diff --git a/contrib/single_file_transcoder/examples/testcard.png b/contrib/single_file_transcoder/examples/testcard.png
index 1f048e6..9291070 100644
--- a/contrib/single_file_transcoder/examples/testcard.png
+++ b/contrib/single_file_transcoder/examples/testcard.png
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diff --git a/encoder_lvl_vs_perf.png b/encoder_lvl_vs_perf.png
index 032feae..5972ec7 100644
--- a/encoder_lvl_vs_perf.png
+++ b/encoder_lvl_vs_perf.png
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diff --git a/webgl/gltf/preview.png b/webgl/gltf/preview.png
index 238067e..ced1794 100644
--- a/webgl/gltf/preview.png
+++ b/webgl/gltf/preview.png
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diff --git a/webgl/texture/preview.png b/webgl/texture/preview.png
index 1107e61..f1c1a6b 100644
--- a/webgl/texture/preview.png
+++ b/webgl/texture/preview.png
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