WIP TileView
- Add mouseBtnUp virtual method and mouseBtnUpInternal to NodeUi
- Add mouseCapture to NodeUi for drag tracking across frames
- Remove clip parameter from mouseMotionInternal
- Add Render::getMousePos() helper for UI coordinate conversion
- Replace inline mouse pos calculations with getMousePos()
- Add uiMode toggle in gameplay (B key) with SDL relative mouse toggle
- Add right mouse button and mouse up handling in gameplay
- Add mouseCapture to Gameplay for UI drag events
- Add TileView scrolling via right mouse button drag
- Move inventory TileView to scene root instead of player node
- Add design doc mockup updates