Rationalise the mechanism in do_text_internal for providing the 'lpDx'

array to ExtTextOut:
 - move it inside the new big loop (this should fix a potential bug
   whereby the DBCS handling altered some elements of it but the loop
   did not actually step along it)
 - initialise it more sensibly
 - rename it to lpDx rather than IpDx, since as far as I can tell the
   latter name was derived from a misreading of the former in the
   Windows API docs.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9066 cda61777-01e9-0310-a592-d414129be87e