nir/xfb: handle arrays and AoA of basic types

nir/xfb: handle arrays and AoA of basic types

On OpenGL, a array of a simple type adds just one varying. So
gl_transform_feedback_varying_info struct defined at mtypes.h includes
the parameters Type (base_type) and Size (number of elements).

This commit checks this when the recursive add_var_xfb_outputs call
handles arrays, to ensure that just one is addded.

We also need to take into account AoA here

v2: use glsl_type_is_leaf from nir_types (Timothy Arceri)

v3: simplified aoa check, without the need ot using glsl_type_is_leaf,
    using glsl_types_is_struct (Timothy Arceri)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>