return null on getContext('webgl') and getContext('experimental-webgl')

Currently, we return a dummy `WebGLRenderingContext`. But this type implements
virtually nothing. From MDN: "If the context identifier is not supported null is
returned."

Now, normally we like to move things along as much as we can. Returning
`WebGLRenderingContext` lets us know the next thing the code needs. But in this
case, it seems to be problematic. At least some code is defensive around a null
value from getContext('webgl'), but once we return an instance, it expects it
to work correctly. This is what causes https://github.com/features/copilot to
enter an endless JS loop.

src/browser/webapi/canvas/WebGLRenderingContext.zig

@@ -204,7 +204,8 @@ pub const JsApi = struct {
     pub const getSupportedExtensions = bridge.function(WebGLRenderingContext.getSupportedExtensions, .{});
 };
 
-const testing = @import("../../../testing.zig");
-test "WebApi: WebGLRenderingContext" {
-    try testing.htmlRunner("canvas/webgl_rendering_context.html", .{});
-}
+// getContext('web-gl') currently returns null, so this cannot be tested
+// const testing = @import("../../../testing.zig");
+// test "WebApi: WebGLRenderingContext" {
+//     try testing.htmlRunner("canvas/webgl_rendering_context.html", .{});
+// }

src/browser/webapi/element/html/Canvas.zig

@@ -85,9 +85,17 @@ pub fn getContext(self: *Canvas, context_type: []const u8, frame: *Frame) !?Draw
             break :blk .{ .@"2d" = ctx };
         }
 
+        // We only stub a tiny slice of the WebGL API (getParameter,
+        // getExtension, getSupportedExtensions). Real WebGL consumers like
+        // Three.js immediately call createTexture/createBuffer/etc. and
+        // throw `TypeError: e.createTexture is not a function`. Pretending
+        // WebGL works until the first non-stubbed call is the worst of both
+        // worlds: pages that have an error boundary above the WebGL widget
+        // catch the throw, reset, re-render, and loop forever.
+        // Spec-correct signal for "no WebGL" is null, so apps that check
+        // (Three.js does) can degrade gracefully.
         if (std.mem.eql(u8, context_type, "webgl") or std.mem.eql(u8, context_type, "experimental-webgl")) {
-            const ctx = try frame._factory.create(WebGLRenderingContext{});
-            break :blk .{ .webgl = ctx };
+            return null;
         }
         return null;
     };