webgl: Throw an error on readPixels(width < 0 || height < 0)

Otherwise gleam will try to allocate a negative size area for the
result, and we'll panic with oom.

components/script/dom/webglrenderingcontext.rs

@@ -1777,6 +1777,10 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
             _ => return Ok(self.webgl_error(InvalidOperation)),
         }
 
+        if width < 0 || height < 0 {
+            return Ok(self.webgl_error(InvalidValue));
+        }
+
         let (sender, receiver) = ipc::channel().unwrap();
         self.ipc_renderer
             .send(CanvasMsg::WebGL(WebGLCommand::ReadPixels(x, y, width, height, format, pixel_type, sender)))