Merge branch 'master' into viewport ...

# Conflicts:
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.h
#	examples/vulkan_example/imgui_impl_glfw_vulkan.cpp

Examples: OpenGL3: Removed unnecessary backup/restore of GL_ELEMENT_ARRAY_BUFFER_BINDING since this is part of the VAO state. (#1836)

Examples: iOS: Fixed missing call to CreateContext() + various shallow coding style tweaks. (#1835)

Examples: Comments about GLSL version not being the same as GL context version. (#1828)

Examples: GLFW: Made it possible to Shutdown/Init the backend again (by reseting the time storage properly). Changelog. Added to Vulkan example. (#1827)

Examples: GLFW* Make subsequent window creation possible.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	imgui.cpp
#	imgui.h

Examples: OpenGL3: Trying to make the call to glBindSampler optional to make the example code easier to use/share with different context version. (#1806)

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/sdl_opengl2_example/imgui_impl_sdl_gl2.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	imgui.cpp
#	imgui.h

Viewport, Platform: Cleaned up xxx_UpdateMouseCursor() functions to make them more consistent. (#1542)

Viewport, Platform: Win32: Fixed handling of io.WantSetMousePos + added a bunch of comments. GLFW, SDL2: Added handling of io.WantSetMousePos. (#1542)

Viewport: Changed Monitor field to use Pos+Size (more consistent), changed FullMin,FullMax to MainPos,MainSize. Made main viewport accessible in PlatformIO on first frame. Fixed casing of ImGuiViewportFlags_TopMost flag. (#1542)

TODO list update + Changelog and comments for #1803.

Fixed clipboard paste memory leak in SDL examples. (#1803)

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_dx9.cpp
#	imgui.cpp
#	imgui.h

Examples: DirectX9: Comments, updated changelog, notes about usage of imconfig.h (#1790, #1687)

add renderstate D3DRS_SHADEMODE for Dx9 impl ...

add backuping transform for Dx9 impl
(#1790, #1687)

Viewport, Platform: Refresh monitor list (win32, glfw) + avoid calling GetWindowFocus before platform window creation to not require of backend to null-check things inconsistently. (#1542)

Viewport: Removed unnecessary fields (now that the coordinate system is consistent accross viewports): MouseRefPrevViewport, MouseClickedPosViewportId. (#1542)

Added IMGUI_CHECKVERSION() macro to compare version string and data structure sizes in order to catch issues with mismatching compilation unit settings. (#1695, #1769)

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/allegro5_example/imgui_impl_a5.cpp
#	examples/allegro5_example/imgui_impl_a5.h
#	examples/allegro5_example/imgui_impl_allegro5.cpp
#	examples/allegro5_example/imgui_impl_allegro5.h
#	examples/allegro5_example/main.cpp
#	examples/imgui_impl_allegro5.cpp
#	examples/imgui_impl_allegro5.h
#	imgui.cpp
#	imgui_internal.h

Platform: Added platform_io.Platform_SetWindowFocus, Platform_GetWindowFocus function wrappers (unused yet). Exact specs tbd because our simplified concept of focus doesn't necessary match the more complex OS native concepts. (#1542)

Viewport: Fixed mouse hover flicker on mouse button release frame after moving a window. (#1542) ...

Platform: Clarifying some comments in back-ends.

Viewport, Platform: Fixed IME positioning for multi-viewport. Moved API from ImGuiIO to ImGuiPlatformIO. Because it is extremely unlikely to people redefined this API manually the moving-forward-breakage is ok. (#1542) ...

SDL2 ime support under Win32 never worked properly because of SDL interferences.

Viewport: Clamp windows within monitors + fallback rescue window when it is out of sight (e.g. removed monitor, changed resolution) + Win32: declare primary monitor at the beginning of the list. (#1542)

Viewport: Added ImGuiConfigFlags_ViewportsNoMerge flag (to enforce a platform window for all floating windows) + minor tidying up and addition of non-functional wip code.

Viewport, Platforms: SDL: Fixed initial focus click being ignored as a mouse button. GLFW: Fixed mouse cursor support for multi-viewport with GLFW 3.3 (current master) - somehow doesn't appear to work with GLFW 3.2 but I'm not too fussed about it. (#1542)

Viewport, Platform: Added work area in ImGuiPlatformMonitor. Renamed fields. (#1542)

Viewport: Switched to using unified platform-absolute mouse coordinates, which simplify lots of problems and simplify/reduce tricky conversions, makes ImVec2 less ambiguous. Fixed various viewport/windowing/popups/synchronization bugs. Settings on host-viewport are stored as relative (made settings decently compatible between viewport enable/disabled settings). Merged ImGuiViewport::Pos and ::PlatformPos. Tweaked thumbnails. Better, smaller code. (#1542)

Viewporrt. Examples: DirectX10,11: Make the platform SetWindowSize handler not crash on failure to resize, which could happen (rarely) on invalid data or bug in the code.