[Feature] Allow arbitrary GuiContainers to be scrollable

[oznogon]

Scrolling elements are currently implemented at the GuiElement subclass level (i.e. GuiListbox, GuiScrollText). So is clipping. Implement clipping and scrolling as options for either the standard GuiContainer or as a GuiContainer subclass.

This might be accomplishable using glScissor, similar to how the radar is clipped, plus translation of the child elements on scroll events and storing and reapplying scroll offset on hide/show events.

Having scrolling GuiContainers within scrolling GuiContainers might require intersecting scissor areas.

[oznogon]

#2383 similarly implements a scrollable GuiElementListbox, which instead hide()s and show()s full child elements depending on whether they're within a scroll range scaled to the rect.