faster image compression when alwaysKeepResolution is set to true

[imtiazShakil]

When alwaysKeepResolution field is set to true, existing code tries to decrease the quality linearly.

• For large images (specially png files), this takes a lot of time and (see #219 )
• sometimes the compressed file size fails to be lower than maxSizeMB (see #176 )

To improve the performance in this special case, we used binary search on quality based on the notion

• higher quality leads to larger image
• lower quality leads to smaller image

Our target is

• find the highest quality where the compressed image file size is lower than maxSizeMB in O(log n) time

Demo improvements:

• added a new checkbox alwaysKeepResolution, that allows to change quality without reducing resolution
• show the time it took to do the compression