ui: scroll texture instead of rendering texture

[deltabeard]

On scrolling, the UI should not have to be re-rendered. Instead, the previously rendered texture should be moved. The selection box should be rendered separately using the hitbox of the currently selected UI element.

[deltabeard]

Text already uses cached textures, and the other elements being drawn on scroll are really fast to render (coloured squares for tiles). Scrolling a rendered texture probably isn't required.