Breaking changes for most users! ...

Added an extra 'void* renderer_user_data' parameter to ImDrawCallback (stored in ImDrawCmd::UserCallback). This allows custom rendering back-ends to pass custom local rendering information to the callback.
IF YOU USE AN OLD RENDERER BACK-END OR COPIED A RENDERER BACK-END BEFORE 1.71, YOUR COMPILATION WILL FAIL ON THIS LINE:
     pcmd->UserCallback(cmd_list, pcmd);
You can fix it by adding a trailing NULL parameter:
     pcmd->UserCallback(cmd_list, pcmd, NULL);
If your back-end needs to support multiple versions, you can use a '#if (IMGUI_VERSION_NUM >= 17004)' test.

IO: changed AddInputCharacter(unsigned short c) signature to AddInputCharacter(unsigned int c). ...

Examples/Backends: Don't filter characters under 0x10000 before calling io.AddInputCharacter(), the filtering is done in io.AddInputCharacter() itself. This is in prevision for fuller Unicode support. (#2538, #2541)

Examples: DirectX10/11/12, Allegro, Marmalade: Render functions early out when display size is zero (minimized). (#2496)

ImDrawData: Added FramebufferScale field (currently a copy of the value from io.DisplayFramebufferScale). ...

This is to allow render functions being written without pulling any data from ImGuiIO, allowing incoming multi-viewport feature to behave on Retina display and with multiple displays. If you are not using a custom binding, please update your render function code ahead of time, and use draw_data->FramebufferScale instead of io.DisplayFramebufferScale. (#2306, #1676)
Examples: Metal, OpenGL2, OpenGL3: Fixed offsetting of clipping rectangle with ImDrawData::DisplayPos != (0,0) when the display frame-buffer scale scale is not (1,1). While this doesn't make a difference when using master branch, this is effectively fixing support for multi-viewport with Mac Retina Displays on those examples. (#2306) Also using ImDrawData::FramebufferScale instead of io.DisplayFramebufferScale.
Examples: Clarified the use the ImDrawData::DisplayPos to offset clipping rectangles.

About, IO: Added io.BackendPlatformName, io.BackendRendererName for informational/QA purpose.

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

Examples: Cast to ImTextureID instead of void* when assigning to TexId. Applied to all examples. (#2015)

Examples: Comments, bits of documentation

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

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)