ImDrawList: Added Clone() helper function.

ImVector: resize() / push_back() use memcpy() as we are not supposed to be constructor/destructor/operator aware at all. (If we do need to rework ImVector). In turn this will allow us to define assign/copy operators.

ImVector: Using value_type consistently. Spacing.

Update TODO list, minor fix in Readme, comments, Clang -Weverything warning fix in imgui_draw.cpp under Windows.

Viewport, Platform: Internal renaming to use more consistent (OsDesktopPos > PlatformPos). (#1542)

Viewport, Platform: SDL: Makes the CreateViewport function restore current GL context so in theory it is free from side-effect. That said, it seems like there is a bug in SDL because our CreateViewport (currently in Render(), not for long) have affect a jerky side-effect if SDL_GL_MakeCurrent() is called before Render(). (#1542)

Viewport: Removed back-end renderer code that implied we need to clear with WindowBg color. Added NoRendererClear flag. (#1542, #1042)

Examples Refactor: GLFW: Explicit functions to init GLFW with OpenGL or Vulkan since we cannot read the api hints from glfw.

Viewport, Platform, Examples: Changes to resizing flow + restored support for Platform events affecting the ImGui windows (so Decorated windows are functional). (#1542, #1042) .. ...

SDL: Added platform move/resize/close support.
GLFW: Added platform move/resize support. Moved Close to use callback for consistency.
Win32:
Vulkan: Fixed resize support.
Naming is WIP "PlatforrmRequestXXX" is too ambiguous. Basically we either have a ImGui->Platform flow or a Platform->ImGui flow. Working a bigger refactor now.

Metrics: Displaying all draw lists of a window (if it uses layer 0 and has already been appended to earlier in the frame).

Examples: SDL: Fix for versions olders than 2.0.6

Examples: SDL,GLFW,Vulkan: The Platform<>Renderer link is handled by SDL/GLFW platforms, both can compile without Vulkan headers, SDL+Vulkan is now on part with GLFW+Vulkan (aka broken the same way!). (#1542)

Examples: Using draw_data->DisplaySize, followup to c50198d. Fix Vulkan secondary viewport rendering. SDL+Vulkan: Matched changes. Fix vcprojs. (#1542, #1042)

Viewport: Fixed software mouse cursor from appearing in all multiple lists simultaneously. (#1542)

Viewport: Obsoleted io.DisplayPos (which was a Viewport branch thing), added ImDrawData::DisplayPos, DisplaySize (#1542) wip

Viewport: Removed unnecessary function hoop.

Merge branch 'master' into viewport

Moved IM_NEW, IM_DELETE helpers to imgui.h for convenient use by back-ends and user code.

ImVector: Spacing, assert tweaks

Misc: Fix 111ea7a, GetStyleColorName() would assert, removed removed ImGuiCol defines (too much risk/complexity trying to silently ignore it).

Contributing: tweaks, mention screenshots more prominently.

Examples, Viewport: Vulkan: Experiment (broken) multi-viewport support, merging code from ParticlePeter branches. (#1542, #1042)

Merge branch 'master' into viewport (inc Vulkan resize fix merge #1042)

Examples: Vulkan: Passing Queue, QueueFamily to binding. Fixed scissor (fixed in master). ImGui_ImplVulkanH_DestroyWindowData() waits for device to be idle. (#1042)

Misc comments, todos + reintroduced removed ImGuiCol defines under !IMGUI_DISABLE_OBSOLETE_FUNCTIONS wrap.

Examples: DPI: Minor warning fix. (#1676)

Examples: Vulkan: Only resize swap chain and framebuffer once. (#1042)

Examples: Renamed glfw error callback for clarity.

Fixed resize window validation errors with removing frame prerender once and present last but one frame functionality. Frame rate is still similar.

Updated templates, added pull request template.