Tweak changelog + tweak internal render helper functions.

Renamed ImFont::GlyphRangesBuilder to ImFontGlyphRangesBuilder. Keep redirection typedef (will obsolete).

Changed ImGuiCol_ChildBg to (0,0,0,0) in Dark style instead of (1,1,1,0), to match other styles. Shouldn't have any effect for the end-user.

Added BETA api for Tab Bar/Tabs widgets. (#261, #351) (merged this feature from the from Docking branch so it can be used earlier as as standalone feature) ...

- Added BeginTabBar(), EndTabBar(), BeginTabItem(), EndTabItem(), SetTabItemClosed() API.
- Added ImGuiTabBarFlags flags for BeginTabBar().
- Added ImGuiTabItemFlags flags for BeginTabItem().
- Style: Added ImGuiCol_Tab, ImGuiCol_TabHovered, ImGuiCol_TabActive, ImGuiCol_TabUnfocused, ImGuiCol_TabUnfocusedActive colors.
- Demo: Added Layout->Tabs demo code.
- Demo: Added "Documents" example app showcasing possible use for tabs.

Version 1.67 WIP (again, this time IMGUI_VERSION_NUM has leeway for another hot-fix).

Version 1.66b (will revisit how to change IMGUI_VERSION_NUM across versions, this commit reduces the numerical IMGUI_VERSION_NUM compared to the commit on Nov 22).

Fixed a text rendering/clipping bug introduced in 1.66 (on 2018-10-12, commit ede3a3b9) that affect single ImDrawList::AddText() calls with single strings larger than 10k. Text/TextUnformatted() calls were not affected, but e.g. InputText() was.

Include <alloca.h> also on macOS (and osxcross) (#2218)

Version 1.67 WIP

Version 1.66

Fix Clang/GCC's null-conversion warning: (#2081) ...

The warning is "implicit conversion of NULL constant to 'unsigned int' [-Wnull-conversion]". Happens when ImTextureID is not a void*, but unsigned int, for example.

Fixes crash introduced in previous commit 9cf94d5.

RenderText(), InputTextMultiline(): Optimization for large text by using memchr, wmemchr, wcschr when appropriate.

ImDrawList: Fixed AddConvexPolyFilled() undefined behavior when passing points_count smaller than 3, in particular, points_count==0 could lead to a memory stomp if the draw list was previously empty.

Internals: Replace unsigned short with ImWchar when dealing with character storage. (#2078)

Version 1.66 WIP

Version 1.65

Renamed stb_truetype.h to imstb_truetype.h, stb_textedit.h to imstb_textedit.h, and stb_rect_pack.h to imstb_rectpack.h. (#1718, #2036) ...

If you were conveniently using the imgui copy of those STB headers in your project, you will have to update your include paths.
The reason for this change is to avoid conflicts for projects that may also be importing their own copy of the STB libraries. Note that imgui's copy of stb_textedit.h is modified.

Refactor: Tweaked and improved the sectioning to facilitate grepping/moving around and applied to all files. (#2036)

Version 1.65 WIP

Version 1.64

Tweak comments and indexes

Refactor: Moved Scrollbar function from imgui.cpp to imgui_widgets.cpp, added file index (#2036)

Version 1.64 WIP

Version 1.63

Internals: Tidying up, moving helpers to imgui_internal.h (in prevision of 1.64 refactor)

ImFontAtlas Comments (#2042) + added ImGuiFontAtlas_None for consistency.

ImDrawList: Large text passed to AddText() are being scanned for their end in order to avoid pre-reserving too many vertices.

ImDrawList: Fixed clipping of leading lines above the clipping rectangle from counting in the worst case vertices reservation. (fix code added in #200!)

Internals: Extracted RenderMouseCursor() out of EndFrame(). Moved to imgui_draw.cpp along with RenderArrowPointingAt(). Comments. (#2013)