WIP: RFC: charging_sdl: make battery icon take up only half of the screen

Some benefits of making the battery icon take up only half of the screen:

• more space for extra UI elements (such as showing the voltage, an auto-poweroff warning, etc)
• fits better with charging icons from other projects, like Android or stock bootloaders

There's a bug with the font rendering: sometimes the font won't actually appear in the middle. I'm not yet sure what's causing this.