ossfuzz: improve code coverage

fix .gif pattern matching
add netpbm test images derived from the public domain pngsuite
added more image types (downloaded in Dockerfile)

tests/ossfuzz.sh

@@ -17,9 +17,13 @@ cp $SRC/stb/tests/stb_png.dict $OUT/stb_png_read_fuzzer.dict
 
 tar xvzf $SRC/stb/jpg.tar.gz --directory $SRC/stb/tests
 tar xvzf $SRC/stb/gif.tar.gz --directory $SRC/stb/tests
+unzip    $SRC/stb/bmp.zip    -d $SRC/stb/tests
+unzip    $SRC/stb/tga.zip    -d $SRC/stb/tests
 
-find $SRC/stb/tests -name "*.png" -o -name "*.jpg" -o -name ".gif" | \
+find $SRC/stb/tests -name "*.png" -o -name "*.jpg" -o -name "*.gif" \
-     xargs zip $OUT/stbi_read_fuzzer_seed_corpus.zip
+                 -o -name "*.bmp" -o -name "*.tga" -o -name "*.TGA" \
+                 -o -name "*.ppm" -o -name "*.pgm" \
+    | xargs zip $OUT/stbi_read_fuzzer_seed_corpus.zip
 
 echo "" >> $SRC/stb/tests/gif.dict
 cat $SRC/stb/tests/gif.dict $SRC/stb/tests/stb_png.dict > $OUT/stbi_read_fuzzer.dict