wacom: skip over the prefix when handling pen data

The prefix (4 bytes on the spark/slate) is some data that we don't know yet.
It *could* be the size of a tablet/drawing in LE byte order: 14434x29794
although that doesn't quite match the Intuos Pro data where the prefix is
different.

Either way, having this be an opcode of 0x3800 was probably just coincidence.
Let's skip over the header normally and assume that once we have a prefix, we
have a drawing. This opcode never occurs a second time within the same
drawing.

tuhi/wacom.py

@@ -589,19 +589,19 @@ class WacomProtocolBase(WacomProtocolLowLevelComm):
     def parse_pen_data_prefix(self, data):
         expected_prefix = b'\x62\x38\x62\x74'
         prefix = data[:4]
+        offset = len(prefix)
         # not sure if we really need this check
         if bytes(prefix) != expected_prefix:
             logger.debug(f'Expected pen data prefix {expected_prefix} but got {prefix}')
-            return False
+            return False, 0
 
-        return True
+        return True, offset
 
     def parse_pen_data(self, data, timestamp):
         '''
         :param timestamp: a tuple with 9 entries, corresponding to the
         local time
         '''
-        offset = 0
         x, y, p = 0, 0, 0
         dx, dy, dp = 0, 0, 0
 
@@ -610,17 +610,16 @@ class WacomProtocolBase(WacomProtocolLowLevelComm):
         drawing = None
         stroke = None
 
-        if not self.parse_pen_data_prefix(data):
+        success, offset = self.parse_pen_data_prefix(data)
+        if not success:
             return []
 
+        drawing = Drawing(self.device.name, (self.width, self.height), timestamp)
+        drawings.append(drawing)
+
         while offset < len(data):
             bitmask, opcode, raw_args, args, offset = self.next_pen_data(data, offset)
-            if opcode == 0x3800:
-                logger.info(f'beginning of sequence')
-                drawing = Drawing(self.device.name, (self.width, self.height), timestamp)
-                drawings.append(drawing)
-                continue
-            elif opcode == 0xeeff:
+            if opcode == 0xeeff:
                 # some sort of headers
                 time_offset = int.from_bytes(raw_args[4:], byteorder='little')
                 logger.info(f'time offset since boot: {time_offset * 0.005} secs')