Cap PixelBuffer.from_bytes dimensions at 64k x 64k

Following review feedback, the reader now rejects a header claiming a
width or height beyond 65536, guarding against malformed byte streams
that would otherwise trigger an unreasonable allocation. The writer
stays full 32-bit ('unlimited'), and native little-endian order and the
transparency mask are kept as-is. Adds Ruby and Python tests for the
oversized-header rejection.

Ref: https://github.com/KLayout/klayout/pull/2387#issuecomment-4984941409

testdata/python/layPixelBuffer.py

@@ -78,6 +78,10 @@ class LAYPixelBufferTests(unittest.TestCase):
     with self.assertRaises(Exception):
       pya.PixelBuffer.from_bytes(data[:-4])
 
+    # a header beyond the 64k x 64k limit is rejected
+    with self.assertRaises(Exception):
+      pya.PixelBuffer.from_bytes(struct.pack("=II", 65537, 1))
+
 
 # run unit tests
 if __name__ == '__main__':