Compile fix
Compile fix

Implicit default constructor + member initializers for nested types can only be
used in member functions. Such code seems to be fine with GCC 4.8 but not with
clang.

ukoeln-vis/ctpperf,tu500/visionaray,ukoeln-vis/ctpperf,szellmann/visionaray,szellmann/visionaray,tu500/visionaray