To make room for two more 16-bit skinning-related values in the
PhongDrawUniform -- joint offset for multidraw and per-instance joint
count for instancing.
Originally I made those full 32 bits because I wanted to provide an
option to have a 64-bit light mask there instead of an offset + count.
But the mask is not really driver-friendly as going over the bits and
skipping zeros was behaving like if each pixel was affected by 64 lights
-- an absolute perf nightmare. I might reconsider this again later, but
for now that doesn't seem to make sense, and I need to put the
skinning-related info somewhere without inflating the per-draw uniform
by another 16-byte vector.
Vladimír Vondruš