LibWeb: Implement CanvasRenderingContext2D.createPattern()

This is a first pass at implementing CRC2D.createPattern() and the
associated CanvasPattern object. This implementation only works for a
few of the required image sources [like CRC2D.drawImage()], and does
not yet support transforms. Other than that it supports everything
else (which is mainly the various repeat modes).

Userland/Libraries/LibWeb/HTML/CanvasPattern.idl

@@ -0,0 +1,5 @@
+[Exposed=(Window,Worker)]
+interface CanvasPattern {
+  // opaque object
+  // FIXME: undefined setTransform(optional DOMMatrix2DInit transform = {});
+};