fast thumbnail creation for jpeg images by using DCT coefficients

[jwurzer]

Is your feature request related to a problem? Please describe.
Create a thumbnail without full decoding of the jpeg image. Necessary for jpeg images which have no embedded EXIF thumbnail.

Describe the solution you'd like
Collects the DCT coefficients from the image, and performs a rescaling operation on them. The DCT coefficient is normally used as the base coefficient for an 8x8 block of pixels. Treat it as one pixel. As a result, a (computationally free) thumbnail exactly 1/8th the size of the original image is created.

Describe alternatives you've considered
Encode the whole jpeg file and resize it to a thumbnail. The problem is it needs more performance.

Additional context
There is already a solution with EPEG using libjpeg (see https://github.com/mattes/epeg ). But the lib libjpeg is necessary. Fine would be a solution with stb image.