Fix offset error in StdIn buffering
Fix offset error in StdIn buffering

When the stdin handler returns a string with a newline character,
the `GhostscriptStdIO` class incorrectly removes the number of
bytes requested from ghostscript rather than the number of bytes
processed before the early return.

Instead of removing `count` characters from the buffer string, remove
only `position` characters.
jhabjan/Ghostscript.NET