Implement get_*_draw_list and DrawList drawing functions

[tangmi]

Closes #220

• Merge 1imgui::render::DrawList1 with 1imgui::WindowDrawList1
• Rename WindowDrawList to DrawList and make [Window|Foreground|Background]DrawList into a wrapper for DrawList
• Add 'ui lifetime to DrawData and *DrawList
• Make all DrawList methods take &mut self instead of &self

Breaks DrawData/DrawList API?

• Rename window_draw_list to draw_list, but for now, this keeps the diff sane.
• Expose DrawList, ImColor publicly
• Path API?

[tangmi]

@Gekkio I believe this PR should be a safe implementation of the drawlist apis.

Notably:

• All DrawList functions take &mut self, as they mutate the underlying draw list
• DrawList is bound to 'ui now
• add_circle, etc return a builder with a mutable reference to the DrawList with a new lifetime pub fn add_circle<'a, ...>(&'a mut self, ...) -> Circle<'ui, 'a>

Edit: looks like methods like with_clip_rect and channels_split have borrow issues, as they require &mut DrawList, but also take a delegate that requires &mut DrawList as well. A workaround is forcing people to call Push and Pop individually, but that doesn't follow the "safe"-ness pattern of this library. Another option would be to pass the &mut DrawList to the delegate as a parameter, but that may be confusing to callers seeing a borrow of a DrawList they otherwise already have. I could also make a type that is a ChannelSplitToken-like thing that DerefMuts into DrawList, so people could do something like:

let draw_list = ...;
{
    let draw_list_channels = draw_list.channel_split(2);
    draw_list_channels.add_line(...);
    draw_list_channels.set_current(0);
    ....
    // channel split is popped here
}

thoughts?

[saucesaft]

What can be done to implement this functions into imgui-rs? Anything i can help with?

[tangmi]

@saucesaft Aside from the merge conflict and failing CI checks, the bit in my last comment about lifetimes regarding with_clip_rect and channels_split I think is still true. Another thing I recall when using this branch is that while there is WindowDrawList, ForegroundDrawList, and BackgroundDrawList, there is no easy way to write code abstracted across them (this might be as simple as making DrawList pub.

I think @Gekkio is pretty busy (I am too!) and I believe neither of us have looked at this since the summer! Feel free to clean this up and re-propose the changes, I think some test cases that sanity check the API would catch things like in the second half of my last comment.

[Boscop]

Any update on this? :)

In my use case I need access to the drawlist..

[tangmi]

Closing, as I believe this was addressed in #414!