Fix in ImageResizer to keep Image Density values (DPI/PPI)
Fix for JpegEncoder to rewrite DPI info

Jpeg files can store this information in JFIF and EXIF. The EXIF values
are kept to see the image metadata as it was shot. The Jpeg JFIF header
for APP0 will be modified if it exists or added if the Image was not a
JFIF. This library always adds JFIF headers to Jpeg files.
Fix for hierarchical Jpeg files

These kinds of files weren't supported because they have multiple Jpeg
frames. These frames are likely used by digital cameras to be able to
produce fast renders on limited hardware and bandwidth. The JFIF
Extension specification reports that the JPEG decoder will be
responsible for decoding the appropriate frame depending on the context.

This commit always decodes the biggest frame into the resulting image.
If developers want control of this feature a property on the decoder
that defines the behavior should suffice.