Various zealous warning fixes (Clang).

Removed reliance on ImU64 type for the ImDrawList assert. (#1184)

ImVector: Spacing.

ImVector: insert() uses grow_capacity() - had inconsistent resize policy

ImVector: Added assignments and = operators + comments.

Examples: DirectX: Using IM_ARRAYSIZE()

Various zealous warning fixes (thanks Clang).

Examples: Added null_example/ which is helpful for quick testing on multiple compilers/settings without relyong on graphics library.

Columns: Made PixelsToOffsetNorm() properly symetrical to OffsetNormToPixels() (#125)

Columns: Fixed dragging when using a same of columns multiple times in the frame. (#125)

Drag and Drop: Disable tracking mouse button ownership when an external drag source is active, to make it easier to achieve drag and drop over multiple OS windows. (#143)

ImDrawList, Font: Fixed bug introduced in 531c11d (#1519)

Internals: String functions uses size_t in their signature

ImDrawList: Added ImDrawListFlags for AA settings. ImDrawList doesn't directly depends on GImGui anymore.

Begin: Moved modal darkening draw block

ImDrawList: Removed 'bool anti_aliased = true' final parameter of ImDrawList::AddPolyline() and ImDrawList::AddConvexPolyFilled(). Anti-aliasing is controlled via the regular style.AntiAliased flags.

ImDrawList: Small refactor toward removing dependency on GImGui + PushClipRectFullscreen now correctly uses data provided by imgui which can takes account of DisplaySize + Removed static variable in PathArcToFast() which caused linking issues to some.

ImDrawList: Comments

Active Modal window always set the WantCaptureKeyboard flag (#744)

BeginPopup: Moved flags into individual BeginPopupXXX calls/implementations and outside of BeginPopupex(). Removed _NoResize flag which is extraneous with AlwaysAutoResize.

Merge branch 'ibachar-master'

InputText: renamed ImGuiInputTextFlags_DisableUndo to ImGuiInputTextFlags_NoUndoRedo (#1506, #1508)

Added a flag for text input to disable undo / redo

Columns: Fixed previous commit (wrong chunk commited) (#125)

Columns: Internal: Columns have their no set of flags so NoResize can be setup by internal code. (#125)

Drag and Drop: Added ImGuiDragDropFlags_SourceExtern to facilitate interfacing with WM_DROPFILES (#143)

Drag and Drop: Source can also inhibit the preview on target, useful for extern sources that only exists for one frame. (#143)

Drag and Drop: Renamed to ImGuiDragDropFlags_SourceNoAutoTooltip to ImGuiDragDropFlags_SourceNoPreviewTooltip (#143)

Using the ImGuiWindowFlags_NoScrollWithMouse flag on a child window forwards the mouse wheel event to the parent window, unless either ImGuiWindowFlags_NoInputs or ImGuiWindowFlags_NoScrollbar are also set. (#1380, #1502)

Fixed unused variable warning.