Exclude .png files from git auto-EOL (#2161)

Fix #1995 for users with the old git version

.gitattributes

 # Use Unix line endings in all text files.
 * text=auto eol=lf
+*.png -text
 
 # Tell git which symlinks point to files, and which ones point to directories.
 # This is revelevant for Windows only, and requires git >= 2.19.2 to work.