Grayscale images always use standard quant tables

[erikandre]

Describe the bug

Due to a check in quant.cc, any grayscale image will always use the standard quant tables even if the std_quant feature is not enabled. This is resulting in significantly worse quality/compression rates.

To Reproduce

Compress any grayscale (single channel) image using cjpegli, observe that you get the same output whether you use the "--std_quant" option or not.

Expected behavior

Difference in file size / quality when "--std_quant" is used

Environment

Should repro for any build config

[erikandre]

Created a PR with a fix: #103