Viewport, Examples: Renamed types used by examples. (#1542)

Viewport, Platform: Refactored platform interface. Removed need to use imgui_internal.h in backends. Split viewport into public facing ImGuiViewport and internal structure. Exposing enough data to provide custom tweaked renderers. Renamed handlers, fixed lots of inconsistencies. (#1542, #1042)

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.h

imgui.h: Various comments and tweaks.

imgui.h: Moved columns lower in the file. Various spacing changes and minor comments.

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.h

ImVector: Added copy/assign operators as helpers to ease cloning of data.

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)