fix: flag used for locked/visible layers

[stephenlacy]

This fixes the flag used for locked vs visible layers, I have a basic psd (PS CC latest) with three layers, one locked, one visible, one not visible. The flags returned look like the following:

Locked layer flags:  00001001 
Visible layer flags: 00001000 
Hidden layer flags:  00001010 

Currently it's looking for the visible flag in position 7, rather than 6.

[pastelmind]

This doesn't match corroborates Adobe's PSD file format docs:

Flags:
bit 0 = transparency protected;
bit 1 = visible;
bit 2 = obsolete;
bit 3 = 1 for Photoshop 5.0 and later, tells if bit 4 has useful information;
bit 4 = pixel data irrelevant to appearance of document

...but its certainly possible (or probable) that the docs are outdated or inaccurate. Oops, I need to wake up before writing replies.

We did plan to eventually rewrite the bit parsing logic, and we'll investigate if we can fix this along the way.

[pastelmind]

Closing in favor of #34