Update cimgui, fixing imgui-rs#97 and imgui-rs#88.

As a side note, the previous version has some compiler warning showing
up during build:

warning: third-party/cimgui/imgui/imgui.cpp: In function ‘void ImGui::RenderTriangle(ImVec2, ImGuiDir, float)’:
warning: third-party/cimgui/imgui/imgui.cpp:3136:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
warning:          r = -r; // ...fall through, no break!
warning:          ~~^~~~

The new version has no warning.

…eightWithSpacing

…reeNodeFlags::AllowItemOvelap

This commit adds the `ImGuiFocusedFlags' input flag that was missing.

Should use `sys::igIsWindowFocused(ImGuiFocusedFlags_RootWindow)`
instead.

Should use `igIsWindowFocused(ImGuiFocusedFlags::RootAndChildWindows)`
instead.

Update struct for ImGuiHoveredFlags.

NB: igIsWindowHovered, and thus igIsRootWindowOrAnyChildHovered, are
broken now because of a bug.
The fix is commited upstream, but not released yet.
cimgui/cimgui@baebcfc

I guess nothing can be done but wait for the next cimgui release.

There is a small change of behaviour: previous height value is not
preserved when igSetNextWindowContentWidth is used. There is no way
around it unless we mess with imgui's internals.

Official Dear ImGui chose this easy solution as well:

    static inline void SetNextWindowContentWidth(float w) { SetNextWindowContentSize(ImVec2(w, 0.0f)); }

https://github.com/ocornut/imgui/blob/429f48bb4fa7fdffdb996e4a697d6df48babe3eb/imgui.h#L1132

This commit does not include deprecation warning.
Indeed, defining a `ImGuiTextBuffer_append` function with a deprecation
warning would require to write a variadic function in rust. However,
this is impossible, and the FFI rust doc says so:
https://doc.rust-lang.org/book/first-edition/ffi.html#variadic-functions

Dear ImGui did not deprecate `append()`, they simply renamed it too.

There is no way I know of to make a deprecation warning in such a case.
Code that uses ImDrawList_AddPolyline or ImDrawList_AddConvexPolyFilled will
break.

Add deprecation warning for each renamed variant.

However, it will just fail to compile if the deprecated variant is used
inside a match statement. It is not possible to make aliases of variants
in Rust.

…erSize, PopupBorderSize

Here is the original commit in Dear ImGui implementating the "No close"
button:

ocornut/imgui@20ba79aa5

Move igCombo{1,2,3} into the combo group along with igBeginCombo and
igEndCombo.

Add binding to igGetDrawListDrawData and igGetOverlayDrawList.

Group functions related to focus management together.

…ctor

Unfortunately, it seems necessary to pull a new dependency, libc, to be able
to use size_t.

Is there any alternative?

…onfig

Follow "keep a changelog" specification.

https://keepachangelog.com/en/1.0.0/

Use WindowBorderSize as replacement for border boolean in window.rs