I added a few improvements to how we load the Photoshop layers:
- When included from TIFF, we forgot to decrease the data_length for
the parts we already read.
- Also when included from TIFF, we need to skip the global layer mask
info.
- The block length for the layer data needs to be a multiple of 4.
- Catch an invalid data condition where block_len is greater than
total_len. This could cause a change total_len to a very large
value, so protect against that.