feat: Server: Add support for previews via sdcpp API (#2093)

Co-authored-by: leejet <leejet714@gmail.com>

include/stable-diffusion.h

@@ -461,6 +461,11 @@ struct ggml_tensor;
 typedef void (*sd_log_cb_t)(enum sd_log_level_t level, const char* text, void* data);
 typedef void (*sd_progress_cb_t)(int step, int steps, float time, void* data);
 typedef void (*sd_preview_cb_t)(int step, int frame_count, sd_image_t* frames, bool is_noisy, void* data);
+
+typedef struct {
+    int sample_pass;
+    int total_steps;
+} sd_preview_info_t;
 typedef bool (*sd_graph_eval_callback_t)(struct ggml_tensor* t, bool ask, void* user_data);
 
 SD_API void sd_set_log_callback(sd_log_cb_t sd_log_cb, void* data);
@@ -469,6 +474,10 @@ SD_API void sd_set_progress_callback(sd_progress_cb_t cb, void* data);
 // negative interval previews only completed logical step -interval. Zero previews the final
 // completed step of the first sampling pass (base-resolution or high-noise).
 SD_API void sd_set_preview_callback(sd_preview_cb_t cb, enum preview_t mode, int interval, bool denoised, bool noisy, void* data);
+// Query from a preview callback. Passes are numbered from 1 since the last
+// sd_set_preview_callback call; total_steps is the actual count in that pass.
+// Both fields are zero before sampling starts.
+SD_API sd_preview_info_t sd_get_preview_info();
 SD_API void sd_set_backend_eval_callback(sd_graph_eval_callback_t cb, void* data);
 SD_API int32_t sd_get_num_physical_cores();
 SD_API const char* sd_get_system_info();