Removed unused code, modified mask to render into a separate scene, a…

…dded additional comments, cleaned up description

sample/stencilMask/fullscreenQuad.vert.wgsl

@@ -7,6 +7,9 @@ struct Uniforms {
   offset_x: f32,
   offset_y: f32,
   radius_scale: f32,
+  sdf_id: u32,
+  scale_to_canvas_x: f32,
+  scale_to_canvas_y:f32,
 }
 
 @group(0) @binding(0) var<uniform> uniforms: Uniforms;
@@ -33,8 +36,12 @@ fn vertexMain(@builtin(vertex_index) VertexIndex : u32) -> VertexOutput {
 
   var output : VertexOutput;
   let mask_offset = vec2f(uniforms.offset_x, uniforms.offset_y);
+  // scale_to_canvas is effectively a transformation that takes our quad positions from NDC space to canvas space.
+  let scale_to_canvas = vec2<f32>(uniforms.scale_to_canvas_x, uniforms.scale_to_canvas_y);
+  let posCS = pos[VertexIndex] * scale_to_canvas;
+  let offsetCS = mask_offset * scale_to_canvas;
   output.Position = vec4(
-    pos[VertexIndex] / uniforms.radius_scale + mask_offset, 
+    posCS * uniforms.radius_scale + offsetCS, 
     0.0, 
     1.0
   );