Fix PreCreateHeader to allow palettes for <=256 colour RISC OS 5 sprites. LoadCoordPair tweaks.
Jeffrey Lee authored
Detail:
  s/vdu/vdudecl - Add a couple more optimisations to LoadCoordPair - use SXTH where possible, and use LDRSH if data is halfword aligned (not currently the case, but may be in future)
  s/vdu/vdugrafj - Fix PreCreateHeader to allow <=256 colour RISC OS 5 sprites to have palettes
Admin:
  Tested on Pandaboard


Version 5.35, 4.79.2.238. Tagged as 'Kernel-5_35-4_79_2_238'
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