Add support for RLE-compressed, 16 bits per channel PSD files.
It doesn't make much sense to me to compress a 16 bpc image using 8-bit RLE encoding. However both GIMP and Krita use RLE to encode 16 bpc PSD images anyway, so adding read support for this format would not hurt.
I've added a single-layer test image as the test case. My implementation is on the ugly side, so any suggestion to improve it will be appreciated.