Wimp_Extend 257
---------------
=> R0 = 257
   R1 = pointer to filename in a buffer to be updated
<= R0 = pointer to buffer passed in
   R1 = 0 (if buffer updated, preserved if not supported by this Wimp)

This SWI is used to append a mode suffix to a filename within a buffer. The
buffer should have space for up to 8 characters in order to allow for future
expansion. The suffixes follow the standard pattern of describing the pixel
sizes in terms of OS units.

Initially, the current mode suffix will be tried. If this does not exist,
the name will be normalised to the lowest pixel size. If the suffix is 11
and this fails then 22 will be tried. If these fail, then the call will fall
back to no suffix. For example, in a mode with pixels using the size 1 OS
unit in the x direction and 4 OS units in the y direction (eigen factors EX0
EY2) the suffixes examined would be 14, 24, 44 and finally falling back to
no suffix.

Clients should check whether R1 has been set to 0 on return. If it has not,
this call is not supported by the WindowManager.

Note: Under Select 3i1 this call was Wimp_Extend 14 but this was found to
      clash with an undocumented call under RISC OS 5. It has been
      reallocated.
