server: fix speculation after an image (#28715)

* server: fix speculation after an image

Pass the actual position to the drafter after an image, instead of the
token count. Affects every drafter, not just DFlash.

* rename draft n_past to pos0

n_past is used to denote number of tokens and this parameter is meant to be a position

tools/server/server-context.cpp

@@ -3028,7 +3028,7 @@ private:
                         common_speculative_get_draft_params(spec.get(), slot.id) = {
                             /* .drafting = */ true,
                             /* .n_max    = */ n_draft_max,
-                            /* .n_past   = */ slot.prompt.n_tokens(),
+                            /* .pos0     = */ slot.prompt.tokens.pos_next(),
                             /* .id_last  = */ slot.sampled,
                             /* .prompt   = */ &slot.spec_prompt,
                             /* .result   = */ &slot.spec_draft,