Update Layer specification

[nightm4re94]

\<layer> All tags shall occur before the first tag so that parsers may rely on having the tilesets before needing to resolve tiles.

• [x] class: The class of the layer (since 1.9, defaults to “”).
• [x] tintcolor: A tint color that is multiplied with any tiles drawn by this layer in #AARRGGBB or #RRGGBB format (optional).
• [x] parallaxx: Horizontal parallax factor for this layer. Defaults to 1. (since 1.5)
• [x] parallaxy: Vertical parallax factor for this layer. Defaults to 1. (since 1.5)