Fixed texture subdata setters.
Fixed texture subdata setters.

Functions for setting 2D subimage of 3D texture / 1D subimage of 2D
texture introduced in 728ddb19a3294e64feba6b600b8770580e3378fa weren't
probably tested at all, as they are ambiguous overloads.

Now using std::enable_if, which works.

MiUishadow/magnum,ashimidashajia/magnum,MiUishadow/magnum,DerThorsten/magnum,ashimidashajia/magnum,MiUishadow/magnum,MiUishadow/magnum,ashimidashajia/magnum,ashimidashajia/magnum,MiUishadow/magnum,DerThorsten/magnum,DerThorsten/magnum,DerThorsten/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum