Remove all ancillary chunks except for tRNS

build/sanitize-png-files.sh

Index: build/sanitize-png-files.sh
diff --git a/build/sanitize-png-files.sh b/build/sanitize-png-files.sh
index 62faf6bd6a080273ba3679a582853376d64ea3a2..f13a191e6127f328ee1fc58092a03cc9da2e9bdf 100755
--- a/build/sanitize-png-files.sh
+++ b/build/sanitize-png-files.sh
@@ -19,8 +19,9 @@ function sanitize_file {
   echo -ne "$1\r"
   local file=$1
   local name=$(basename $file)
-  pngcrush -d $TMP_DIR -brute -reduce -rem text -rem mkBT \
-      -rem mkTS $file > /dev/null
+  # -rem alla removes all ancillary chunks except for tRNS
+  pngcrush -d $TMP_DIR -brute -reduce -rem alla $file > /dev/null
+
   mv "$TMP_DIR/$name" "$file"
 }