Replacing texture tx_raindrop_01.dds causes rain to disappear

Replaced the raindrop texture using tx_raindrop_01.dds from Skies .IV mod (http://www.nexusmods.com/morrowind/mods/43311/?).

During a rainstorm, rain is not visible.

No other modifications enabled.

Running OpenMW nightly b94a4c1c

Raindrop texture is replaced and works normally in MGE.

(RM-3465 from redmine: created on 2016-07-09 by Andrew O'Connor, , closed on 2016-07-12 by nobody, duplicates #3404)