vbo: fix out-of-bounds array access

vbo: fix out-of-bounds array access

The exec->vtx.inputs[] array was being written past its end.  This was
clobbering the following vbo_exec_context::eval state.  Probably not noticed
since evaluators and immediate mode rendering don't happen at the same time.

Fixed the loop in vbo_exec_vtx_init().
Changed the size of the vbo_exec_context::vtx.arrays[] array.
Added a bunch of debug-build assertions.

Issue found by Vinson Lee.