Moving window doesn't use accumulating MouseDelta so straying out of boundaries keeps moved window at the same spot.

Examples: DirectX10,DirectX11: Moved call to OMSetRenderTargets() in main loop so example code can integrate more nicely with other code.

Misc comments, removed duplicated IM_ARRAYSIZE macro in imgui_demo.cpp (it is now declared in imgui.h)

Examples: Comments, synched some minor drift between examples + stronger suggestion to use StyleColorsDark().

Renamed the emblematic ShowTestWindow() function to ShowDemoWindow(). Kept redirection function (will obsolete).

ImVector: Revert 6172e932 actually problematic with our current use (because we don't construct the instances).

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

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

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.

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

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)