[COG] Mixed Raw and Overview data compression ?

[Vincent Sarago]

What do I mean about `mixed` compression ? 
JPEG (or WEBP) compression are great for storage saving (and also for small http request) but they are `bad` because they are not lossless (WEBP can be set to be lossless but compression is then not as great) so my idea is to package both lossless and non-lossless in one COG. e.g keeping high resolution data to a ZSTD/DEFLATE compression but the overview in JPEG.

Why ? 
For web application we mostly want fast tile serving while for processing we will mostly want to read full resolution lossless data, a file with mixed compression could make both world happy ? 

The gain won’t be astonishing but might be worth a try ? 

Vincent