Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_sdl.cpp

Examples: Comments about GLFW/SDL versions

Moved guidelines to issue #2261 to Pin and increase visibility for now.

Renamed extra_flags to flags in InputXXX parameters.

InputFloat: When using ImGuiInputTextFlags_ReadOnly the step buttons are disabled. (#2257)

Happy new year! & comments

Comments: fixed missing line in the "how a simple rendering function may look like" section (#2258)

Error recovery: Extraneous/undesired calls to End() are now being caught by an assert in the End() function itself at the call site (instead of being reported in EndFrame). Past the assert, they don't lead to crashes any more. Missing calls to End(), pass the assert, should not lead to crashes any more, nor to the fallback/debug window appearing on screen. (#1651).

Tweaked asserts

Viewport: Misc comments following user feedbacks..

Build fix for master.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_sdl.cpp

Examples: SDL: Avoid testing for SDL_GetKeyboardFocus() on Android and iOS (like Emscripten). (#421)

Examples: SDL: Fixed compilation for SDL 2..0.3 and less (running on our test servers) and clarified a bit of the messy situation. Followup to 8dd83c5. (#1542, #2117)

Merge branch 'master' into viewport + added style tweak block when viewports are enabled ...

# Conflicts:
#	examples/example_glfw_opengl3/main.cpp
#	examples/example_sdl_opengl3/main.cpp
#	examples/example_win32_directx11/main.cpp

Examples: Setting up style before bindings, so in complex binding (vulkan/dx12) it isn't miles away from the context creation.

Examples: Viewport: Moved the "make current GL context" to reduce the amount of call and hopefully be more explicit about viewport enabled vs disabled requirements. (#1542)

Viewport: Fixed not clearing request flags in main viewport, which led some back-end (SDL) to break on resize as PlatformRequestResize would stay true forever and inhibit new sizes passed to AddUpdateViewport(). (#1542)

Examples: SDL: SDL_GetMouseState() seems problematic, movements feels laggy in the non-viewport code path. (#1542, #2117)

Made it illegal to call Begin("") with an empty string. This somehow accidentally worked before but had various undesirable side-effect as the window would have ID zero. In particular it is causing problems in viewport/docking branches.

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui.h

Viewport: Corrected/clarified comments. Moved RenderPlatformWindowsDefault() next to UpdatePlatformWindow(). Removed unnecessary flag check.

IO: Realigned all fields, very minor comments change. This is nearly a no-op if you don't ignore Spaces.

IO: Added BackendPlatformUserData, BackendRendererUserData, BackendLanguageUserData void* for storage use by back-ends. (#2004 + for cimgui)

Changed ImGuiCol_ChildBg to (0,0,0,0) in Dark style instead of (1,1,1,0), to match other styles. Shouldn't have any effect for the end-user.

Fixed using SetNextWindowPos() on a child window (which wasn't really documented) position the cursor as expected in the parent window, so there is no mismatch between the layout in parent and the position of the child window. Demo tweak and adding some child window stuff

Various user-facing comments

Tidying up README, moved entries to FAQ, updated screenshots, removed comma in title.

Fix merge issue on master.

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp