Fix stamp tool cursor positioning to match anchor point

[bengotow]

Summary

Fixed the stamp tool to position dropped sprites at the cursor location by computing the drag offset based on the sprite's anchor point, matching the visual feedback shown in the cursor.

Changes

• Import DEFAULT_APPEARANCE_INFO from sprite module for fallback appearance data
• Refactored drag offset calculation to handle three cases:
  • Drag-and-drop: Use the offset stored in dataTransfer (existing behavior)
  • Stamp tool: Compute offset dynamically based on the sprite's anchor point and current scale
  • Default: Fall back to center-based offset for other tools
• For stamp tool, resolve the correct appearance info by:
  • Looking up the anchor actor's character and appearance if stamping an actor
  • Looking up the character and its default appearance if stamping a character
  • Using DEFAULT_APPEARANCE_INFO as fallback
• Calculate screen-space offset accounting for anchor position, cell size, and current zoom scale

Implementation Details

The stamp tool now computes dragLeft and dragTop as the distance from the sprite's top-left corner to its anchor center in screen pixels. This ensures that when a sprite is dropped, its anchor point aligns with the cursor position, matching the visual representation shown in the cursor preview.