glsl ubo/ssbo: Add lower_buffer_access class

glsl ubo/ssbo: Add lower_buffer_access class

This class has code that will be shared by lower_ubo_reference and
lower_shared_reference. (lower_shared_reference will be used to
support compute shader shared variables.)

v2:
 * Add lower_buffer_access.h to makefile (Emil)
 * Remove static is_dereferenced_thing_row_major from
   lower_buffer_access.cpp. This will become a lower_buffer_access
   method in the next commit.
 * Pass mem_ctx as parameter rather than using a member variable (Iago)

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>