Undo modification of ContentsRegionRect.Max, too many side-effects (undo dff07836)

Fix selectable/tree node not reaching right-side of contents size when horizontal scrolling is active and no explicit size is known

Fixed GetWindowContentRegionMax() being off by ScrollSize amount when SizeExplicit is set + caching ContentsRegionRect.

Relates to horizontal scrollbar, explicit contents size

TreeNodeEx(): ImGuiTreeNodeFlags_AlwaysOpen->ImGuiTreeNodeFlags_Leaf, + added ImGuiTreeNodeFlags_Bullet (#324, #581)

Demo: fixed multi-selection tree nodes demo to not replace selection when clicking on single-item that's already part of selection (#581)

Minor sizing refactor, should be no-op. Making it a commit for further bisection since sizing code is super brittle. (#668)

Resizing window doesn't rely on MouseDelta anymore, but rather recompute expected size based absolute mouse coords. (#668)

Storing ActiveIdClickOffset to generalize pattern already used by
columns.

Fix minor typo in examples/README.txt

ImFontAtlas: Tweak to allow MergeMode to apply on a font that isn't the previous one, by setting the DstFont field.

Including limits.h again to get INT_MAX, assuming previous report of missing limits.h was erroneous (#1, yes, issue ONE!)

ImGuiListClipper new version, detect height automatically, fix compatibility with SetScrollPosHere (#662)