Set layer flags bit3 (Photoshop 5.0+) so PS reads tagged blocks correctly

PS-generated PSDs always set flags bit 3 (0x08) to signal Photoshop 5.0+
format where the layer extra data contains tagged blocks. Our layers had
flags=0x00, so Photoshop treated them as pre-5.0 records and failed to
read the extra data / layer pixels, showing every layer as transparent
(only ape_2 happened to render). Verified against the reference PSD.

ppl2psd.py

@@ -472,7 +472,9 @@ def _layer_record(top, left, bottom, right, name, opacity, visible, channels) ->
     rec += b"8BIM" + b"norm"
     rec += struct.pack(">B", opacity)
     rec += struct.pack(">B", 0)          # clipping
-    rec += struct.pack(">B", 0 if visible else 2)  # flags: bit1 = hidden
+    # flags: bit3 = 1 for Photoshop 5.0+ (tells PS the extra data contains
+    # tagged blocks); bit1 = hidden
+    rec += struct.pack(">B", 0x08 | (0 if visible else 0x02))
     rec += struct.pack(">B", 0)          # filler
     # extra data
     extra = bytearray()