View: add fn for toggling optimize and force texture caching (#1136)

* TextInput: expose the location of the caret/cursor

in absolute window-relative coordinates.

THis allows, for ex, a widget to be placed relative to the
current location of the text being inputted by the user.

* View: add fn for toggling `optimize` and force texture caching

Add CachedView fns that make it easier to control its parameters:
* set_optimize(cx, ViewOptimize): switch optimize mode at runtime and
  allocate the draw_list on demand. Previously `optimize` was set once from
  `texture_caching` and never reset, so a view couldn't toggle between direct
  and texture-cached rendering per frame.
* set_texture_max_height(Option<f64>): cap the Texture-mode render turtle's
  height so a tall Fit-height cached view can't allocate a render target past
  the GPU's max texture size (was a hard MTLTextureDescriptor abort once
  content exceeded 16384px). None (default) leaves it uncapped; content past
  the cap is clipped. Only affects Texture mode.
* redraw_texture_cache() / force_texture_redraw: force one offscreen
  re-render after a content repopulate or an optimize-mode flip. The
  rect-based will_redraw check can't see a content change on a recycled or
  toggled view, so without this it would composite a stale texture.
* view_size is now updated in every optimize mode, not just draw-list modes.
  The None (direct-render) path previously left it stale, so a view toggled
  None<->Texture sized its next offscreen turtle from an old height and
  clipped/mis-positioned its content.