Graphics: Only check for threading issues once graphics is initialized

[matham]

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

This fixes an issue, where if you created graphics instructions before kivy gl thread was initialized (which seems to happen with ScrollView), it would the error that you're accessing the graphics from the wrong thread. This fixes so it only does it once the thread has been initialized.

While technically this would allow people to create graphics from another thread before the kivy thread was initialized and it wouldn't warn about it. But it shouldn't happen often and this would still warn for the most common situation, while not warning on false positive.