remove magic numbers

Author: adroitwhiz

src/RenderWebGL.js

@@ -1454,7 +1454,12 @@ class RenderWebGL extends EventEmitter {
         twgl.bindFramebufferInfo(gl, skin._framebuffer);
 
         // Limit size of viewport to the bounds around the stamp Drawable and create the projection matrix for the draw.
-        gl.viewport(240 + bounds.left, 180 - bounds.top, bounds.width, bounds.height);
+        gl.viewport(
+            (this._nativeSize[0] * 0.5) + bounds.left,
+            (this._nativeSize[1] * 0.5) - bounds.top,
+            bounds.width,
+            bounds.height
+        );
         const projection = twgl.m4.ortho(bounds.left, bounds.right, bounds.top, bounds.bottom, -1, 1);
 
         // Draw the stamped sprite onto the PenSkin's framebuffer.