Squashed 'external/imgui/' content from commit 02e9b8cac

git-subtree-dir: external/imgui
git-subtree-split: 02e9b8cacde285c5f9040adf212703cc3810bd97

misc/single_file/imgui_single_file.h

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+// dear imgui: single-file wrapper include
+// We use this to validate compiling all *.cpp files in a same compilation unit.
+// Users of that technique (also called "Unity builds") can generally provide this themselves,
+// so we don't really recommend you use this in your projects.
+
+// Do this:
+//    #define IMGUI_IMPLEMENTATION
+// Before you include this file in *one* C++ file to create the implementation.
+// Using this in your project will leak the contents of imgui_internal.h and ImVec2 operators in this compilation unit.
+
+#ifdef IMGUI_IMPLEMENTATION
+#define IMGUI_DEFINE_MATH_OPERATORS
+#endif
+
+#include "../../imgui.h"
+#ifdef IMGUI_ENABLE_FREETYPE
+#include "../../misc/freetype/imgui_freetype.h"
+#endif
+
+#ifdef IMGUI_IMPLEMENTATION
+#include "../../imgui.cpp"
+#include "../../imgui_demo.cpp"
+#include "../../imgui_draw.cpp"
+#include "../../imgui_tables.cpp"
+#include "../../imgui_widgets.cpp"
+#ifdef IMGUI_ENABLE_FREETYPE
+#include "../../misc/freetype/imgui_freetype.cpp"
+#endif
+#endif