PSD layer count may be negative

[radarhere]

Resolves #5612

https://www.adobe.com/devnet-apps/photoshop/fileformatashtml/#50577409_pgfId-1031423

Layer count. If it is a negative number, its absolute value is the number of layers and the first alpha channel contains the transparency data for the merged result.

But at the moment, we're reading it in as H, unsigned short.

Pillow/src/PIL/_binary.py

Lines 70 to 71 in 696b82e

	def i16be(c, o=0):
	return unpack_from(">H", c, o)[0]

Pillow/src/PIL/PsdImagePlugin.py

Line 23 in 696b82e

from ._binary import i16be as i16

Pillow/src/PIL/PsdImagePlugin.py

Line 182 in 696b82e

ct = i16(read(2))

This PR changes it to signed.

[hugovk]

Thanks!