Fix Frozen line pattern when switching artists

[joshlory]

selectPattern() calls setPattern(), which clears out the pattern if it's not loaded yet. Since we just started loading and haven't yielded, there's no way the pattern could be loaded yet. Use a promise to select the pattern once it's loaded.

code-dot-org/apps/src/turtle/turtle.js

Lines 1427 to 1433 in fe41e8a

	if (this.loadedPathPatterns[pattern]) {
	this.currentPathPattern = this.loadedPathPatterns[pattern];
	this.isDrawingWithPattern = true;
	} else if (pattern === null) {
	this.currentPathPattern = new Image();
	this.isDrawingWithPattern = false;
	}