OptionalhandlerAssigned by the owning handler on registration; available in
load/open (for example this.handler.fetch(...)).
Fetches (typically via this.handler.fetch) and produces the resource, then invokes the callback.
OptionalopenOptional. Called by the default
ResourceHandler#open when parsers are registered. Handlers that override open may call
it with an extended signature - for example the texture handler calls
open(url, data, device, textureOptions) on its parsers.
Returns true if this parser can handle the described resource. Parsers are consulted newest-first; the first to return true is used.