FBX: Fix fbx2json tool for 7500 FBX files (FBX2016).

Based on patch from T49822, thanks a bunch for investigation.

Change is actually pretty limited - 7500 now uses some uint64 instead of
uint32 integers as some root element info (like size etc.), and
accordingly raised the 'stop nested data' marker from 13 to 25 NULL
bytes.

Next step: check if we can make actual importer load FBX2016 files that
simply!

io_scene_fbx/fbx2json.py

@@ -75,9 +75,9 @@ import zlib
 
 # at the end of each nested block, there is a NUL record to indicate
 # that the sub-scope exists (i.e. to distinguish between P: and P : {})
-# this NUL record is 13 bytes long.
-_BLOCK_SENTINEL_LENGTH = 13
-_BLOCK_SENTINEL_DATA = (b'\0' * _BLOCK_SENTINEL_LENGTH)
+_BLOCK_SENTINEL_LENGTH = ...
+_BLOCK_SENTINEL_DATA = ...
+read_fbx_elem_uint = ...
 _IS_BIG_ENDIAN = (__import__("sys").byteorder != 'little')
 _HEAD_MAGIC = b'Kaydara FBX Binary\x20\x20\x00\x1a\x00'
 from collections import namedtuple
@@ -88,6 +88,8 @@ del namedtuple
 def read_uint(read):
     return unpack(b'<I', read(4))[0]
 
+def read_uint64(read):
+    return unpack(b'<Q', read(8))[0]
 
 def read_ubyte(read):
     return unpack(b'B', read(1))[0]
@@ -137,16 +139,34 @@ read_data_dict = {
     }
 
 
+# FBX 7500 (aka FBX2016) introduces incompatible changes at binary level:
+#   * The NULL block marking end of nested stuff switches from 13 bytes long to 25 bytes long.
+#   * The FBX element metadata (end_offset, prop_count and prop_length) switch from uint32 to uint64.
+def init_version(fbx_version):
+    global _BLOCK_SENTINEL_LENGTH, _BLOCK_SENTINEL_DATA, read_fbx_elem_uint
+
+    assert(_BLOCK_SENTINEL_LENGTH == ...)
+    assert(_BLOCK_SENTINEL_DATA == ...)
+
+    if fbx_version < 7500:
+        _BLOCK_SENTINEL_LENGTH = 13
+        read_fbx_elem_uint = read_uint
+    else:
+        _BLOCK_SENTINEL_LENGTH = 25
+        read_fbx_elem_uint = read_uint64
+    _BLOCK_SENTINEL_DATA = (b'\0' * _BLOCK_SENTINEL_LENGTH)
+
+
 def read_elem(read, tell, use_namedtuple):
     # [0] the offset at which this block ends
     # [1] the number of properties in the scope
     # [2] the length of the property list
-    end_offset = read_uint(read)
+    end_offset = read_fbx_elem_uint(read)
     if end_offset == 0:
         return None
 
-    prop_count = read_uint(read)
-    prop_length = read_uint(read)
+    prop_count = read_fbx_elem_uint(read)
+    prop_length = read_fbx_elem_uint(read)
 
     elem_id = read_string_ubyte(read)        # elem name of the scope/key
     elem_props_type = bytearray(prop_count)  # elem property types
@@ -198,8 +218,7 @@ def parse(fn, use_namedtuple=True):
             raise IOError("Invalid header")
 
         fbx_version = read_uint(read)
-        if fbx_version >= 7500:
-            raise IOError("Unsupported FBX version (%d), binary format is incompatible!" % fbx_version)
+        init_version(fbx_version)
 
         while True:
             elem = read_elem(read, tell, use_namedtuple)