DRTVWR-476: Fix glVertexAttrib{IPointer,PointerARB}() OpenGL calls.

VS 2017 complains about the same thing that clang does: casting S32 to GLvoid*
can't possibly produce a valid pointer value because S32 can't fit a whole
64-bit pointer. To appease it, not only must we use reinterpret_cast, but we
must first cast S32 to intptr_t and then reinterpret_cast THAT.