Safari workaround breaks use of preinitialized GL contexts in library_webgl.js

Somewhere between 2.0.15 and 2.0.17 code was added at line [586](https://github.com/emscripten-core/emscripten/blob/d5b212dd0c185d2cba6b08115c29228bdc4c38a8/src/library_webgl.js#L586) of createContext() in library_webgl.js to workaround some Safari issue. This code tries to dereference the canvas argument to createContext and thus breaks use of preinitialized GL contexts. When using those you pass `null` as the canvas parameter.

The code that handles preinitialized contexts is at line [621](https://github.com/emscripten-core/emscripten/blob/d5b212dd0c185d2cba6b08115c29228bdc4c38a8/src/library_webgl.js#L621). Presumably the fix is as simple as moving the Safari workaround to the else after line 621, the not-preinitialized case. But I don't have enough familiarity with the webgl library to feel comfortable providing a PR.

This has broken my JS KTX v2 decoder (including Basis Universal) and WebGL uploader. My users are naturally complaining to me. Please fix it as soon as possible.