WIP #38: Support for >4GB images

- Introduce PBuffer type to replace PByteArray[Index] pattern which is limited to ~2GB elements. PBuffer uses pointer arithmetic (POINTERMATH directive) and falls back to PAnsiChar on older Delphi versions.
- Update buffer offset variables from Integer/LongInt to PtrInt for proper 64-bit offset calculations (needed due to Pascal quirk that result of big 32 bit operands is not promoted to 64 bits).

Author: galfar