Precision improvements to reduce high MLD issues

[x] Changes made in deindex_sph_idx_fx and ivas_dirac_dec_output_synthesis_process_subframe_psd_ls_fx to improve precision
[x] Improved precision for hStereoDft->smooth_res_nrg_fx and hStereoDft->smooth_dmx_nrg_fx buffers
[x] Added checks to prevent saturation for the buffers hHQ_core->old_out_fx and hHQ_core->old_out_LB_fx

• EVS 26.444 BE test passes.
• decoder pytests:
  MLD scores:
  Original inputs: 562 failed, 374 skipped
  MLD > 10: 1
  MLD > 5 : 13
  MLD < 5 : 548
  +10dB inputs: 562 failed, 374 skipped
  MLD > 10: 32
  MLD > 5 : 89
  MLD < 5 : 441
  -10dB inputs: 562 failed, 374 skipped
  MLD > 10: 0
  MLD > 5 : 5
  MLD < 5 : 557
• encoder pytests results:
  MLD scores:
  Original inputs: 562 tests:
  MLD > 10: 18
  MLD > 5 : 67
  MLD < 5 : 477
  +10dB inputs: 562 tests:
  MLD > 10: 69
  MLD > 5 : 122
  MLD < 5 : 371
  -10dB inputs: 562 tests:
  MLD > 10: 5
  MLD > 5 : 24
  MLD < 5 : 533