This replaces the usage of FIELD_OFFSET for dynamic indexing into array fields.
Sadly GCC has broken __builtin_offsetof and they don't seem to intend to fix it.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942
- Make sure the DWLP_* values are correct on _WIN64
- Don't use the DWL_* constants, they are not portable. Enforce this by removing them entirely from winuser.h
- Make sure Get/SetWindowLong*Ptr* is used and pointers are not truncated to LONG
