ImDrawList: Added ImDrawListFlags for AA settings. ImDrawList doesn't directly depends on GImGui anymore.

TODO.txt

@@ -30,12 +30,10 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
 !- scrolling: allow immediately effective change of scroll after Begin() if we haven't appended items yet.
  - scrolling/clipping: separator on the initial position of a window is not visible (cursorpos.y <= clippos.y). (2017-08-20: can't repro)
 
- - drawlist: move Font, FontSize, FontTexUvWhitePixel inside ImDrawList and make it self-contained (apart from drawing settings?)
- - drawlist: make it easier to toggle AA per primitive, so we can use e.g. non-AA fill + AA borders more naturally
  - drawlist: end-user probably can't call Clear() directly because we expect a texture to be pushed in the stack.
  - drawlist: maintaining bounding box per command would allow to merge draw command when clipping isn't relied on (typical non-scrolling window or non-overflowing column would merge with previous command).
- - drawlist: avoid passing null (-9999,+9999) rectangle to end-user, instead perhaps pass rectangle based on io.DisplaySize?
  - drawlist: primtiives/helpers to manipulate vertices post submission, so e.g. a quad/rect can be resized to fit later submitted content, _without_ using the ChannelSplit api
+ - drawlist: make it easier to toggle AA per primitive, so we can use e.g. non-AA fill + AA borders more naturally
  - drawlist: non-AA strokes have gaps between points (#593, #288), especially RenderCheckmark().
  - drawlist: would be good to be able to deep copy a draw list (ImVector= op?).
  - drawlist/opt: AddRect() axis aligned pixel aligned (no-aa) could use 8 triangles instead of 16 and no normal calculation.