限制裁剪框放大时超出画布范围

- 双指缩放裁剪框时添加边界检测
- 裁剪框超出边界时停止放大，保持在画布内

gallery/src/main/java/cc/winboll/studio/gallery/CropCanvasView.java

@@ -414,8 +414,15 @@ public class CropCanvasView extends View {
                     newWidth = Math.max(minSize, Math.min(newWidth, canvasBounds.width()));
                     newHeight = newWidth / coverRatio;
                     
-                    cropRect.set(centerX - newWidth / 2, centerY - newHeight / 2,
-                               centerX + newWidth / 2, centerY + newHeight / 2);
+                    float newLeft = centerX - newWidth / 2;
+                    float newTop = centerY - newHeight / 2;
+                    float newRight = centerX + newWidth / 2;
+                    float newBottom = centerY + newHeight / 2;
+                    
+                    if (newLeft >= canvasBounds.left && newRight <= canvasBounds.right &&
+                        newTop >= canvasBounds.top && newBottom <= canvasBounds.bottom) {
+                        cropRect.set(newLeft, newTop, newRight, newBottom);
+                    }
                     
                     initialSpan = span;
                     invalidate();