In order to be used for OpenGL (right now for ARB_gl_spirv).

This commit adds two new structures:

  * nir_xfb_varying_info: that identifies each individual varying. For
    each one, we need to know the type, buffer and xfb_offset

  * nir_xfb_buffer_info: as now for each buffer, in addition to the
    stride, we need to know how many varyings are assigned to it.

For this patch, the only case where num_outputs != num_varyings is
with the case of doubles, that for dvec3/4 could require more than one
output. There are more cases though (like aoa), that will be handled
on following patches.

v2: updated after new nir general XFB support introduced for "anv: Add
    support for VK_EXT_transform_feedback"

v3: compute num_varyings beforehand for allocating, instead of relying
    on num_outputs as approximate value (Timothy Arceri)

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