reset the position of the current character in the input

Fixes # .

[Describe your changes and what you've already done to test it.]

Commit 1e905cb introduces bug in readSTL.

• when is_stl_binary(...) is called, the position of the current character for std::istream input is accidentally updated because of input.read(...). So, I just add input.seekg(...) to reset the position.

Check all that apply (change to [x])

• All changes meet libigl style-guidelines.
• Adds new .cpp file.
• Adds corresponding unit test.
• This is a minor change.