IDL contains typedefs nested in interface

The file `specs/1.0.3/webgl.idl` contains typedefs inside interface definitions. This is not permitted by the IDL grammar, and prevents some IDL tools (specifically, webidl2.js) from parsing the file.

The typedefs are (with line numbers):

```
519 typedef (ArrayBuffer or ArrayBufferView) BufferDataSource;
```

```
646 typedef (ImageData or
647 HTMLImageElement or
648 HTMLCanvasElement or
649 HTMLVideoElement) TexImageSource;
```

Inspect specs/1.0.3/webgl.idl around lines 519 and 646-649, then use webidl2.js to reproduce the parsing failure. Determine how the typedef declarations should be represented outside the interface definitions, and verify that the corrected file parses successfully.

Written by the indexing model from the issue text.