Examples: Add missing include guards (#2166)

Examples: Using "dear imgui" terminology in all examples headers/comments + fix minor typo.

Examples: OpenGL: Added support for glew and glad OpenGL loaders out of the box. (#2001, #2002). Changelog, tweaks, applied changes to SDL+OpenGL3 example.

WIP on configurable OpenGL loader. (#2001, #2002)

imgui_impl_opengl3: Advertised as a ES2/ES3 renderer. Defaults to ES3 on Android. Default on "#version 300 es" on ES 3. (#2002, #1873)

imgui_impl_opengl3: Comments. (#1987)

Examples: OpenGL3: Added shaders more versions of GLSL + made the example app default to GL 3.0 + GLSL 130 (instead of GL 3.2 + GLSL 150) unless on Mac. (#1938, #1900, #1513, #1466, etc.)

Examples: Comments, bits of documentation

Examples: OpenGL3: Remove misleading or unnecessary "#version 150" strings.

Examples: The functions in imgui_impl_xxx.cpp are prefixed with IMGUI_IMPL_API (which defaults to IMGUI_API) to facilitate some uses. (#1888) + Comments in imgui.h

Internals: Added GetItemID(), GetFocusID() for consistency. Made GetActiveID() inline. Comments, fixed typos, demo tweaks.

Examples: Comments to make ImGuiConfigFlags_NoMouseCursorChange more visible (#1027). + Allegro5: Added support for ImGuiConfigFlags_NoMouseCursorChange flag.

Big example binding refactor (manually imported from Viewport branch, stripped out of Viewport code). (#1870) ...

Read examples/README.txt for some details.
ImDrawData: Added DisplayPos, DisplaySize fields honored by all backends (not strictly necessary to honor just now, but doing it to be ahead)