Viewport: Merging fixes + relying on multiple viewport overlaps. Follow-up to previous attempts are reworking the split/merge mechanisms. (#1542)

Viewport: Comment to suggest making WindowBg opaque when viewports are enabled.

Viewport: Fixes moving child menu viewport (fix 379733a).

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_sdl.cpp

Viewport: Fix viewport regression with protruding child window creating their own viewport. (fixes cae4d02, 760c1d95) (#1542)

Examples: SDL: changed the signature of ImGui_ImplSDL2_ProcessEvent() to use a const SDL_Event*. (#2187)

Viewport: Added support for minimized window which caused problem when ImGuiBackendFlags_HasMouseHoveredViewport was not supported. (#1542) + todo

Viewport: Avoid unnecessary reapplying platform pos/size every frame. (#2205) + Fix GLFW ImGui_ImplGlfw_CreateWindow() from not applying position immediately. Clear LastNameHash properly (bug already fixed in Docking branch)

Viewport: Merging/Extracting to/from main host viewport is now performed based on current state rather than triggers. Windows which don't fit in host window are now consistently in their own viewport. Toward simplifying the UpdateSelectWindowViewport() function. Probably broke something.. (#1542)

Viewport: UpdateTryMergeWindowIntoHostViewport() calls BringWindowToDisplayFront() - possible now that BringWindowToFocusFront() is a different function.

Internals: Added ImPool helper data structure (used by testing system, and already in docking branch)

Version 1.67 WIP

Viewport: Stop relying on viewport->CreatedPlatformWindow to create Platform_DestroyWindow and Renderer_DestroyWindow. Explicitly require Platform_GetWindowFocus() to require supporting empty data.

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp

Version 1.66

Examples: SDL2+Vulkan: Fixed application shutdown which could deadlock on Linux + Xorg. (#2181)

Added link to experimental imgui_scoped.h PR/thread (#2197, #2096)

Examples: OpenGL3: More explicit testing for == GL_UPPER_LEFT in case glGetIntegerv(GL_CLIP_ORIGIN is not honored properly. (#2186, #2195) Fix f52f0a5

Nav: Fixed explicit directional input not re-highlighting current nav item if there is a single item in the window and highlight has been previously disabled by the mouse. (#787)

Internals: Using GetOverlayDrawList(ImGuiWindow*) to match syntax of viewport/master branches. Fixed compiling with IMGUI_DEBUG_NAV_RECTS

Removed unnecessary ImGui:: prefixes.

Internals: Added FindWindowByID() helper. (#2190)

Merge branch 'master' into viewport + comments ...

# Conflicts:
#	examples/imgui_impl_opengl3.cpp
#	imgui.cpp

Nav: Fixed an assert in certain circumstance (mostly when using popups) when mouse positions stop being valid. (#2168) + adding a else block to make NavCalcPreferredRefPos() more explicit.

Viewport: BeginMainMenuBar(): explicitly set viewport to avoid creating new one when ImGuiConfigFlags_ViewportsNoMerge is set + misc shallow changes.

Examples: OpenGL3: Added support for GL 4.5's glClipControl(GL_UPPER_LEFT). (#2186)

Made IsWindowFocused() work outside of NewFrame()-EndFrame() and added comments about how ImGuiFocusedFlags_AnyWindow should NOT be used in place of io.WantCaptureMouse. (#2185)

Update README.md

Internals: DragBehavior: Added support for ImGuiDragFlags_Vertical to implement a vertical drag widget (no frontend function provided).

Internals: SliderBehavior: Using axis indexing.