Feature/issue 25

[sigrist]

• Added new settings in LDtkJsonImporter.cs to allow load HD sprites
• Added new LDtkRelativeGetterTilesetTextureHD class to load the assets from alternative path and not from LDtk file
• Updated LDtkBuilderTileset to use HD version based on the new flag and added the pixelPerUnit logic to set the grid size

[sigrist]

This a suggestion pull request for issue #25

[Cammin]

Sorry I haven't checked up on this in a month 😅

I'll try to get an implementation of this in sometime. Thanks for taking the time to develop a contribution 🙂