ZRLE Subencoding 128 incorrectly always assumes 3 bytes per pixel

[paviad]

When processing a ZRLE tile with subencoding 128 - the code incorrectly assumes each pixel is 3 bytes (CPIXEL).

Modified the code to read the correct number of bytes per pixel as pixel data, and then read the run length.

See section 7.7.5 of the RFC

PS. Incorporated this fix into my PR #12