mtmd: support "frame merge" for qwen-vl-based models (#21858)

* feat: add video support for Qwen3.5

* various clean up

* revise the design

* fix llava-uhd case

* nits

* nits 2

---------

Co-authored-by: andrewmd5 <1297077+andrewmd5@users.noreply.github.com>

tools/mtmd/clip.h

@@ -20,6 +20,12 @@ struct clip_image_size {
     bool operator==(const clip_image_size & other) const {
         return width == other.width && height == other.height;
     }
+    bool operator!=(const clip_image_size & other) const {
+        return !(*this == other);
+    }
+    int area() const {
+        return width * height;
+    }
 };
 
 struct clip_image_f32;
@@ -101,6 +107,8 @@ bool clip_is_llava(const struct clip_ctx * ctx);
 bool clip_has_vision_encoder(const struct clip_ctx * ctx);
 bool clip_has_audio_encoder(const struct clip_ctx * ctx);
 
+int clip_model_n_batch_max(const struct clip_ctx * ctx);
+
 std::map<ggml_backend_dev_t, size_t> clip_get_mem_usage(const struct clip_ctx * ctx);
 
 struct clip_cap {