compressor: rename "i" to "inputOffset"
compressor: rename "i" to "inputOffset"

There will soon be more offset variables in this function. Let's
bring more clarity to the table.
terrelln/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard