Use this return type for Matrix4 in-place functions

[TimJentzsch]

Objective

Closes #704.

Consider the following example:

export class Matrix4Ext extends Matrix4 {
  extension(): this {
    // -- Do stuff --
    return this;
  }
}

new Matrix4Ext()
  .multiply(new Matrix4())
  // Error: Property 'extension' does not exist on type 'Matrix4'.
  .extension();

This example doesn't compile, because the subclass type is erased by the Matrix4 return type of the .multiply function.

Solution

Instead of Matrix4, we now use this as return type for functions which return this.
I used the Matrix4 documentation on threejs.org to determine which return types should be this.

[Methuselah96]

Thanks!