Fixes #821

Adds bounded export functionality to export_image() and layered_to_image() with automatic 16k chunking support.

New ExportMode enum:

• EXPORT_SLICED (default) - Exports full terrain, auto-chunked at 16k boundaries
• EXPORT_PER_REGION - Exports each region as a separate file

Modified functions:

• export_image(file_name, map_type, mode) - Added optional mode parameter
• layered_to_image(map_type, bounds) - Added optional Rect2i bounds parameter

Output logging includes:

• Filename
• Size (pixels)
• Position (pixel and world coordinates)

Usage

var data = terrain.get_data()

# Sliced export (default) - auto-chunks if terrain > 16k
data.export_image("res://terrain.exr", Terrain3DRegion.TYPE_HEIGHT)

# Per-region export - one file per region
data.export_image("res://terrain.exr", Terrain3DRegion.TYPE_HEIGHT, Terrain3DData.EXPORT_PER_REGION)

# Direct bounded export
var img = data.layered_to_image(Terrain3DRegion.TYPE_HEIGHT, Rect2i(0, 0, 8192, 8192))

File naming

• Sliced: filename_00_00.exr, filename_01_00.exr, etc.
• Per-region: filename_00_00.exr, filename_00-01.exr (handles negative coords)
• Single chunk/region: no suffix added

Backward compatible

Default parameters maintain existing behavior - no changes required for existing code.