Convert a Tiled blend mode name to a canonical melonJS blend mode.
Tiled uses "add" for additive blending; melonJS renderers accept
"add", "additive", and "lighter" as equivalent. This helper normalizes
the Tiled "add" value to "lighter" for internal consistency.
Returns "normal" when the mode is undefined or explicitly "normal";
all other values are passed through to the renderer (which will fall
back to "normal" for unsupported modes).
Convert a Tiled blend mode name to a canonical melonJS blend mode. Tiled uses "add" for additive blending; melonJS renderers accept "add", "additive", and "lighter" as equivalent. This helper normalizes the Tiled "add" value to "lighter" for internal consistency. Returns "normal" when the mode is undefined or explicitly "normal"; all other values are passed through to the renderer (which will fall back to "normal" for unsupported modes).