Missing IMWRITE_JPEG_SAMPLING_FACTOR constants

[zaquest]

The list of missing constants:

• IMWRITE_JPEG_SAMPLING_FACTOR
• IMWRITE_JPEG_SAMPLING_FACTOR_411
• IMWRITE_JPEG_SAMPLING_FACTOR_420
• IMWRITE_JPEG_SAMPLING_FACTOR_422
• IMWRITE_JPEG_SAMPLING_FACTOR_440
• IMWRITE_JPEG_SAMPLING_FACTOR_444

This makes it impossible to change sampling factor from the default 4:2:0 to turn off chroma down-sampling for example.

[UrielCh]

For now, you can define them in your code and use them directly. Constant values are just numbers.

Alternatively, open a PR if you want to share those constants with the community.