[ios] Groundwork for new Redraw API, refactoring AppState, and bugfixes (#1133)

* fix #1087. the CFRunLoopTimer was never started if the user never changed the controlflow.

* RedrawRequested ordering matches the new redraw api
consistent asserts
lots of appstate refactoring to rely less on unsafe, and hopefully make it easier to maintain

* ios: dpi bugfix. inputs to setContentScaleFactor are not to be trusted as iOS uses 0.0 as a sentinel value for "default device dpi".

the fix is to always go through the getter.

* move touch handling onto uiview

* update changelog

* rustfmt weirdness

* fix use option around nullable function pointers in ffi

* Document why gl and metal views don't use setNeedsDisplay

* change main events cleared observer priority to 0 instead of magic number
log when processing non-redraw events when we expect to only be processing redraw events

CHANGELOG.md

@@ -33,6 +33,10 @@
 - On iOS, add touch pressure information for touch events.
 - Implement `raw_window_handle::HasRawWindowHandle` for `Window` type on all supported platforms.
 - On macOS, fix the signature of `-[NSView drawRect:]`.
+- On iOS, fix the behavior of `ControlFlow::Poll`. It wasn't polling if that was the only mode ever used by the application.
+- On iOS, fix DPI sent out by views on creation was `0.0` - now it gives a reasonable number.
+- On iOS, RedrawRequested now works for gl/metal backed views.
+- On iOS, RedrawRequested is generally ordered after EventsCleared.
 
 # 0.20.0 Alpha 2 (2019-07-09)