[Switch][UI bug] mGBA doesn't update savestate screenshots until you move the cursor over other savestates

[RafaelTrepaUnCarballo]

Steps to reproduce:

1. Create a savestate in slot Add a manpage for mgba. #1
2. Do something to change the screen (like moving or opening a menu) and overwrite that savestate

If you open the save state menu again, the screenshot the emulator shows it's from the previous savestate, not the current one. It won't show the actual screenshot until you move the cursor over the other savestates slots (there's no need of loading them, only show their preview images).

Tested in the slightly outdated 0.10-6982-29115 build, Atmosphère 0.19.2 and firmware 12.0.1