Rationalise the mechanism in do_text_internal for providing the 'lpDx'

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.

[originally from svn r9066]