Fix various issue regarding window size in music mode

[lhc70000]

Description:

Currently, if there's no video track, mpv won't send the video reconfig event and thus windows won't change their sizes accordingly. Info such as aspect ratio and video size will remain to be the last video track. I'm not sure whether it worked well before and it's an internal change in mpv. Now the player remains functional due to an awkward fallback mechanism, and here are some obvious glitches:

• Disable auto music mode switching and open an audio file without album art. The window aspect ratio will be the initial one (16:10) at first, but suddenly jump to 1:1 on resize. This also happens when switching back from music mode.
• Open a landscape or portrait video, switch to music mode, and open another audio without album art. The album art part of the mini player will have a wrong aspect ratio.

In this PR, notifyMainWindowVideoSizeChanged is also called on playback restart if there's no video track to make sure windows receive the resize event. player.originalVideoSize is used to check if there's no video in the mini player. The default window size has been changed to 640x400.

Before merging, extensive tests are required to ensure it didn't break anything.