Add glu::BufferOffsetAsPointer utility

Add glu::BufferOffsetAsPointer utility

Clang 6.x adds a warning that doing arithmetic involving a NULL
pointer is a GNU extension. This affects the common GL pattern
    (deUint8*)DE_NULL + offset
used to pass an offset into a bound buffer object as a client pointer
parameter.

This change replaces that pattern with a call to a new utility
function glu::BufferOffsetAsPointer(), which will hopefully be
warning-free on all compilers.

Change-Id: I8be939297b02c44091441c71ae75e45ceab30639
Components: Framework, OpenGL, AOSP