RangeSelect/MultiSelect: WIP range-select (ref 1861) [rebased]

Version 1.71 + comments

Readme, comments, dear imgui prefixes

Scrollbar overlap an extra WindowBorderSize amount on the left to make all distances consistent. Reverted to BorderSize not affecting work/contents rectangles. Scrollbar, Style: Changed default style.ScrollbarSize from 16 to 14.

Internals: Minor no-op tidying up toward solving the WindowPadding / WindowBorderSize / ScrollbarSize overlapping mess. ...

+ Demo: Use SetScrollY().

TextWrapped, PushTextWrapPos(0.0f) within a window with horizontal scrolling from not covering the full horizontal area (previously only worked with an explicit contents size).

Window rectangles: Changed WorkRect to cover the whole region including scrolling (toward obsolete ContentsRegionRect) + using full WindowPadding*1 padding. ...

Tweaked InnerClipRect.
TreeNode, CollapsingHeader: Fixed highlight frame not covering horizontal area fully when using horizontal scrolling. (#2211, #2579)
TabBar: Fixed BeginTabBar() within a window with horizontal scrolling from creating a feedback loop with the horizontal contents size.
Columns: Fixed Columns() within a window with horizontal scrolling from not covering the full horizontal area (previously only worked with an explicit contents size). (#125)
Demo: Added demo code to test contentsrect/workrect

Renamed ImGuiBackendFlags_HasVtxOffset to ImGuiBackendFlags_RendererHasVtxOffset to match naming convention already used in viewport/docking branch. (#2591) + Fix OpenGL3 code missing flag.

ImDrawList: Added ImDrawCmd::VtxOffset value to support large meshes (64k+ vertices) using 16-bits indices. To enable the feature, the renderer back-end needs to set 'io.BackendFlags |= ImGuiBackendFlags_HasVtxOffset' and honor the ImDrawCmd::VtxOffset field. Otherwise the value will always be zero. This has the advantage of preserving smaller index buffers and allowing to execute on hardware that do not support 32-bits indices. ...

ImDrawList: Added ImDrawCmd::IdxOffset value, equivalent to summing element count for each draw command. This is provided for convenience and consistency with VtxOffset. (#2591)

Added full "Dear ImGui" prefix to the title of "Dear ImGui Demo" and "Dear ImGui Metrics" windows. Shortened amount of nodes in columns>tree demo.

ColorEdit: Fixed the color picker popup only displaying inputs as HSV instead of showing multiple options. (#2587, broken in 1.69 by #2384).

Style: Added style.WindowMenuButtonPosition (left/right, defaults to ImGuiDir_Left) to move the collapsing/docking button to the other side of the title bar.

Fix OuterRectClipped not being clipped correctly, which resulted in child window outside visible bound to not be marked with SkipItems. Broken in b50c61c. ...

+ Comments on InnerClipRect being misleading. Demo: Tweak to sizing of child window in the Layout->Scrolling section.

Fixed GCC mem-access warnings (#2565) ...

+ using "if defined" more consistently for Clang.

Renamed SetNextTreeNodeOpen() to SetNextItemOpen(). Refactored SetNextItemXXX stuff to match SetNextWindowXXX code closely.

Tweak EndGroup() to facilitate fixing #2550 later (currently should have no side-effect0. Demo: Add extra widget to status query test.

Internals: Columns: Renamed fields. Comments and tweak. Moved a demo block.

Version 1.71 WIP + fixed minor typo

Version 1.70

Demo: Improved trees in columns demo. (#2136)

Obsoleted GetContentRegionAvailWidth(), use GetContentRegionAvail().x instead. Kept inline redirection function.

Amend 48a09a7 with changelog, breaking changes, tweak demo code for spacing. (#2518)

Improved algorithm for mitre joints on thick lines

Popups: Closes popup at the time of FocusWindow(). Fixes right-click from closing all popups instead of aiming at the hovered popup level (regression in 1.67's ae76a1fd).

Misc comments, internal renaming, added disable indentation option to Columns demo section.

Added SetNextItemWidth() helper to avoid using PushItemWidth/PopItemWidth() for single items.

Demo: Documents: Fix misusage of ListBoxHeader().

Internals: Columns: Tweaks, renaming. Metrics: Show rectangles for child windows. ...

Renamed SameLine() first parameter.

Merged a bunch of small changes from Docking branch to reduce the difference between branches. ...

Noticeable: horizontal alignment of CloseButton. Menu fill take account of border. Various stylistic tweaks to accomodate other changes in Docking.

Internals, Docs: Added a bunch of clarification about ButtonBehavior in the form of a table (and to facilitate writing tests)