lpDx_maybe was a pointer defined to point at either lpDx itself or
NULL, depending on whether the code decided it needed to pass the lpDx
array of per-character pixel offsets to various functions during
drawing (based in turn on whether the font was variable-pitch).
lpDx is reallocated as necessary, which means lpDx_maybe must be kept
up to date. This was achieved by resetting it to lpDx if it was
already non-NULL.
But lpDx starts out as NULL before the first reallocation, so that
this can't work - it'll be initialised to NULL even if we _did_ want
to use it, and then at the first realloc, it won't be updated!
Before the previous commit turned lpDx from a static into an automatic
variable, this would have been a rare bug affecting only the first
call to the function. Now it will happen all the time, which is
better, because we can notice and fix it.
Replaced lpDx_maybe completely with a boolean flag indicating whether
we should pass lpDx to drawing functions.
322984d635