Add prevent-canvas scriptlet

Prevent usage of specific or all (default) canvas APIs.

Syntax

```text
example.com##+js(prevent-canvas [, contextType])
```

- `contextType`: A specific type of canvas API to prevent (default to all
  APIs). Can be a string or regex which will be matched against the type
  used in getContext() call. Prepend with `!` to test for no-match.

Examples

1. Prevent `example.com` from accessing all canvas APIs

```adblock
example.com##+js(prevent-canvas)
```

2. Prevent access to any flavor of WebGL API, everywhere

```adblock
*##+js(prevent-canvas, /webgl/)
```

3. Prevent `example.com` from accessing any flavor of canvas API except `2d`

```adblock
example.com##+js(prevent-canvas, !2d)
```

References

https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext

assets/resources/scriptlets.js

@@ -68,7 +68,7 @@ function safeSelf() {
             if ( pattern === '' ) {
                 return { matchAll: true };
             }
-            const expect = (options.canNegate === true && pattern.startsWith('!') === false);
+            const expect = (options.canNegate !== true || pattern.startsWith('!') === false);
             if ( expect === false ) {
                 pattern = pattern.slice(1);
             }
@@ -3388,6 +3388,70 @@ function setAttr(
     runAt(( ) => { start(); }, 'idle');
 }
 
+/*******************************************************************************
+ * 
+ * @scriptlet prevent-canvas
+ * 
+ * @description
+ * Prevent usage of specific or all (default) canvas APIs.
+ *
+ * ### Syntax
+ * 
+ * ```text
+ * example.com##+js(prevent-canvas [, contextType])
+ * ```
+ * 
+ * - `contextType`: A specific type of canvas API to prevent (default to all
+ *   APIs). Can be a string or regex which will be matched against the type
+ *   used in getContext() call. Prepend with `!` to test for no-match.
+ * 
+ * ### Examples
+ * 
+ * 1. Prevent `example.com` from accessing all canvas APIs
+ * 
+ * ```adblock
+ * example.com##+js(prevent-canvas)
+ * ```
+ * 
+ * 2. Prevent access to any flavor of WebGL API, everywhere
+ * 
+ * ```adblock
+ * *##+js(prevent-canvas, /webgl/)
+ * ```
+ * 
+ * 3. Prevent `example.com` from accessing any flavor of canvas API except `2d`
+ * 
+ * ```adblock
+ * example.com##+js(prevent-canvas, !2d)
+ * ```
+ * 
+ * ### References
+ * 
+ * https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext
+ * 
+ * */
+
+builtinScriptlets.push({
+    name: 'prevent-canvas.js',
+    fn: preventCanvas,
+    dependencies: [
+        'safe-self.fn',
+    ],
+});
+function preventCanvas(
+    contextType = ''
+) {
+    const safe = safeSelf();
+    const pattern = safe.initPattern(contextType, { canNegate: true });
+    const proto = globalThis.HTMLCanvasElement.prototype;
+    proto.getContext = new Proxy(proto.getContext, {
+        apply(target, thisArg, args) {
+            if ( safe.testPattern(pattern, args[0]) ) { return null; }
+            return Reflect.apply(target, thisArg, args);
+        }
+    });
+}
+
 
 /*******************************************************************************
  *