[css-paint-api] Locally define alpha flag, you can't just magically rely on the inheriting concepts from CanvasRenderingContext2D.

Author: tabatkins

css-paint-api/Overview.bs

@@ -56,7 +56,6 @@ urlPrefix: https://html.spec.whatwg.org/multipage/; type: dfn;
         text: reset the rendering context to its default state
         text: output bitmap
         text: set bitmap dimensions
-        url: concept-canvas-alpha; text: alpha
 urlPrefix: https://tc39.github.io/ecma262/#sec-; type: dfn;
     text: constructor
     text: Construct
@@ -362,6 +361,14 @@ A {{PaintRenderingContext2D}} object has a [=output bitmap=]. This is initialise
 object is created. The size of the [=output bitmap=] is the size of the fragment it is
 rendering.
 
+A {{PaintRenderingContext2D}} object also has an <dfn for="PaintRenderingContext2D">alpha</dfn> flag,
+which can be set to true or false.
+Initially, when the context is created,
+its alpha flag must be set to true.
+When a {{PaintRenderingContext2D}} object has its alpha flag set to false,
+then its alpha channel must be fixed to 1.0 (fully opaque) for all pixels,
+and attempts to change the alpha component of any pixel must be silently ignored.
+
 The size of the [=output bitmap=] does not necessarily represent the size of the actual bitmap
 that the user agent will use internally or during rendering. For example, if the visual viewport is
 zoomed the user agent may internally use bitmaps which correspond to the number of device pixels in
@@ -392,7 +399,7 @@ When the user agent is to <dfn>create a PaintRenderingContext2D object</dfn> for
 |height|, and |paintRenderingContext2DSettings|, it <em>must</em> run the following steps:
     1. Create a new {{PaintRenderingContext2D}}.
     2. [=Set bitmap dimensions=] for the context's [=output bitmap=] to the rounded values of |width| and |height|.
-    3. Set the {{PaintRenderingContext2D}}'s [=alpha=] flag to |paintRenderingContext2DSettings|'s {{alpha}}.
+    3. Set the {{PaintRenderingContext2D}}'s [=PaintRenderingContext2D/alpha=] flag to |paintRenderingContext2DSettings|'s {{alpha}}.
     4. Return the new {{PaintRenderingContext2D}}.
 
 Note: The initial state of the rendering context is set inside the [=set bitmap dimensions=]