title: Reverse a sin and cos transform
body: <p>I'm manipulating a 2D map with basic sin and cos quite successfully to give a 3d perspective.</p>
  
  <p>The issue I'm having is I'm stuck trying to find what is visible in the viewport.
  Effectively doing the transformation in reverse, the output is [0,0] and [width,height] and I want to know where on my map those points lie.</p>
  
  <p>I've got it to the point of trying to reverse:</p>
  
  <pre><code>display_x = (source_x * cos(z)) + (source_y * sin(z))
  display_y = (source_y * cos(z)) - (source_x * sin(z))
  </code></pre>
  
  <p>If I know <code>display_x</code> and <code>display_y</code> and <code>source_x</code> and <code>source_y</code> are the unknowns, how can I solve what the unknowns are?
  My simultaneous equation knowledge is a little rusty.</p>