add missing DOMMatrixInit and DOMPointInit types

this was accidentally relying on people importing ambient DOM
declarations

index.d.ts

@@ -395,6 +395,16 @@ export class JPEGStream extends Readable {}
 /** This class must not be constructed directly; use `canvas.createPDFStream()`. */
 export class PDFStream extends Readable {}
 
+// TODO: this is wrong. See matrixTransform in lib/DOMMatrix.js
+type DOMMatrixInit = DOMMatrix | string | number[];
+
+interface DOMPointInit {
+  w?: number;
+  x?: number;
+  y?: number;
+  z?: number;
+}
+
 export class DOMPoint {
 	w: number;
 	x: number;

lib/DOMMatrix.js

@@ -19,6 +19,8 @@ class DOMPoint {
   }
 
   matrixTransform(init) {
+    // TODO: this next line is wrong. matrixTransform is supposed to only take
+    // an object with the DOMMatrix properties called DOMMatrixInit
     const m = init instanceof DOMMatrix ? init : new DOMMatrix(init)
     return m.transformPoint(this)
   }