Improved integration with the Tiled map editor

Today I finished making some improvements to the engine's built-in integration with the Tiled and the TMX file format:

• Implemented support for nested layer groups
• Fixed a crash that occured on maps with unset background color
• Added support for loading tilesets from external *.tsx files
• Enabled tilesets that use a single spritesheet image (both embedded and external)
• The render layers api (https://blog.ldk.dev/render-layers/) can now be used to set an explicit render order on individual map layers and objects (so you can blend your environment with non-map entities, like characters hiding in the scenery)
• Layers and objects can now be rendered with custom opacity values
• Enabled tint color for adjusting the color of objects and layers

We're getting pretty close to full support for every feature of the TMX map format!

The following features are still unsupported:

• Infinite maps
• Image layers
• Polygon and polyline objects
• Embedded text