vbo: better handling of VBO allocation failures

vbo: better handling of VBO allocation failures

Previously, if we failed to allocate a VBO (either for display list
compilation or immediate mode rendering) we'd eventually segfault
when trying to map the non-existant buffer or in a glVertex/Color/etc
call when we hit a null pointer.

Now we don't try to map non-existant buffers and if we do fail to
allocate a VBO we plug in no-op functions for glVertex/Color/etc
so we don't segfault.