Playback doesn't always play to the end of the loop.

**Describe the bug**
I'm making the assumption that when looping, playback should always play to the end of the loop. It doesn't.

**To Reproduce**
Steps to reproduce the behavior:
1. With looping on, set the start of the loop to be before the end of any audio, and the end of the loop to be later than any of the audio.
2. Place the cursor somewhere in the audio.
3. Start playback.
4. Playback ends up looping from the start of the loop to the end of the audio.
5. Note that if you drag the loop just slightly to move it's position, the playback then loops from the start to the end of the loop.
6. And if you place the cursor between the end of the audio and the end of the loop, playback is interesting.

Another case, probably not often used, but it confused me for some time, thinking that playback had stopped working:
1. With looping on, set the start and end of the loop to be after the audio.
2. Place the cursor somewhere in the audio.
3. Start playback.
4. Nothing happens.

**Expected behavior**
I think the expected behaviour is that playback should play to the end of the loop. But maybe the spec is otherwise.

**Additional information (please complete the following information):**
- Windows 10
- Audacity 3.1.0

Reproduce the two looping cases in Audacity 3.1.0 on Windows 10 using the steps in the report, including loops that extend beyond or sit after the audio. Trace the playback behavior from the relevant playback entry point and compare it with the expected result: playback should reach the loop end, or the intended behavior should be documented if the specification differs.

Written by the indexing model from the issue text.