get rid of UIComponent.mask

Not easily implemented on other platforms and probably only used for scrolling/clipping so should be in a higher-level abstraction.

Also check for scrollRect

Start by locating UIComponent.mask and scrollRect references across the repository, then trace how they support scrolling or clipping and which higher-level abstraction would own that behavior. Done means the proposed removal is implemented without leaving unsupported references or breaking the relevant scrolling and clipping behavior.

Written by the indexing model from the issue text.