feat: add --pingpong flag to giftool

Add pingpong mode to giftool command. When --pingpong flag is used,
the generated GIF will play forward then backward, creating a
back-and-forth looping effect.

Features:
- Add --pingpong option to giftool command
- Support combining with --speed for adjusted playback speed
- Update documentation with new option

Examples:
- giftool [图片] --pingpong
- giftool [图片] --pingpong --speed 2.0

konabot/plugins/image_process/__init__.py

@@ -34,6 +34,7 @@ cmd_giftool = on_alconna(
         Option("-t", Args["length", str]),
         Option("-to", Args["end_point", str]),
         Option("--speed", Args["speed_factor", float], default=1.0, alias=["-s"]),
+        Option("--pingpong", default=False),
     )
 )
 
@@ -46,6 +47,7 @@ async def _(
     length: str | None = None,
     speed_factor: float = 1.0,
     end_point: str | None = None,
+    pingpong: bool = False,
 ):
     ss: None | float = None
     if start_point:
@@ -162,6 +164,16 @@ async def _(
         rframes = rframes[::-1]
         rdur_ms = rdur_ms[::-1]
 
+    # 处理 pingpong 模式
+    if pingpong:
+        # 复制一份反转的帧序列（去掉第一帧避免重复）
+        pingpong_frames = rframes[1:][::-1] if len(rframes) > 1 else rframes[::-1]
+        pingpong_durations = rdur_ms[1:][::-1] if len(rdur_ms) > 1 else rdur_ms[::-1]
+        
+        # 拼接正放和倒放
+        rframes = rframes + pingpong_frames
+        rdur_ms = rdur_ms + pingpong_durations
+
     output_img = BytesIO()
 
     if rframes: