Add RAII / scoped gl::VboMesh attrib mapping

[richardeakin]

Currently you need to call something like:

auto mappedPosIter = mVboMesh->mapAttrib3f( geom::Attrib::POSITION );
// ... modify mapped data buffer
mappedPosIter.unmap();

But it is easy to forget to call the unmap(), and also inconvenient. It would be nice to have something like:

auto mappedPosIter = mVboMesh->scopedMapAttrib3f( geom::Attrib::POSITION );

That automatically unmapped when it went out of scope. Or perhaps a ScopedVboMeshMap, etc. Complications come up around dealing with the different data types, vec2, vec3, etc...