Support clipping

[JD557]

MicroUI has 4 types of operation:

• MU_COMMAND_TEXT
• MU_COMMAND_RECT
• MU_COMMAND_ICON
• MU_COMMAND_CLIP

InterIm already supports text and rectangle, and I think icons are better served by a custom element.

However, there's still the question of clipping, which is required for smooth scrolling inside a box.

I'm not much of a fan of having this as a RenderOp, as it adds some extra complexity to the interpreter - as some backends might not support this natively and it requires keeping some state.

I think it might be viable to move that state logic to the UIState so that:

• Rectangles are already clipped (or ignored) when added to the list of render ops
• Text and Custom ops bring both an area and a clip area
• The draw char conversion ignores chars outside of the clip area.

I'm not sure if I should also add a clip rect to chars, or if I should just keep it simple... maybe I'll add it anyway and let the backend sort that out.