Fix timestamp offset bug in BLFWriter

In the BLF files, the frame timestamp are stored as a delta from the "start time". The "start time" stored in the file is the first frame timestamp rounded to 3 decimals. When we calculate the timestamp delta for each frame, we were previously calculating it using the non-rounded "start time".

This new code, use the "start time" as the first frame timestamp truncated to 3 decimals.

Author: pierreluctg

can/io/blf.py

@@ -530,7 +530,7 @@ def _add_object(self, obj_type, data, timestamp=None):
         if timestamp is None:
             timestamp = self.stop_timestamp or time.time()
         if self.start_timestamp is None:
-            self.start_timestamp = timestamp
+            self.start_timestamp = int(timestamp * 1000) / 1000
         self.stop_timestamp = timestamp
         timestamp = int((timestamp - self.start_timestamp) * 1e9)
         header_size = OBJ_HEADER_BASE_STRUCT.size + OBJ_HEADER_V1_STRUCT.size