exif_process_IFD_TAG() currently accepts a dir_entry, offset_base
and IFDlength. However, it's very hard to follow how these values
are related to each other and the addressable memory region. As we
add additional bounds check, this gets further confused.
One of the basic cases is where dir_entry is in
[offset_base, offset_base+IFDlength), in which case the memory
[dir_entry, offset_base+IFDlength) is valid, but the memory
[offset_base, dir_entry) is not necessarily valid. I wasn't able
to understand what exactly is valid if dir_entry is outside
[offset_base, offset_base+IFDlength)
This patch changes everything to use a struct that separately stores
offset_base and the valid memory region and adds helpers to fetch
offsets and check that pointers are in-bounds.
Closes phpGH-5068.