Fixed using imgui_freetype.cpp in unity builds. (#2302)

docs/CHANGELOG.txt

@@ -40,6 +40,7 @@ Other Changes:
 - ImGuiTextBuffer: Added append() function (unformatted).
 - ImFontAtlas: Added 0x2000-0x206F general punctuation range to default ChineseFull/ChineseSimplifiedCommon ranges. (#2093)
 - ImFontAtlas: FreeType: Added support for imgui allocators + custom FreeType only SetAllocatorFunctions. (#2285) [@Vuhdo]
+- ImFontAtlas: FreeType: Fixed using imgui_freetype.cpp in unity builds. (#2302)
 - Examples: Win32: Using GetForegroundWindow()+IsChild() instead of GetActiveWindow() to be compatible with windows created
   in a different thread or parent. (#1951, #2087, #2156, #2232) [many people]
 - Examples: Win32: Added support for XInput games (if ImGuiConfigFlags_NavEnableGamepad is enabled).

misc/freetype/imgui_freetype.cpp

@@ -244,10 +244,12 @@ namespace
     }
 }
 
+#ifndef STB_RECT_PACK_IMPLEMENTATION        // in case the user already have an implementation in the _same_ compilation unit (e.g. unity builds)
 #define STBRP_ASSERT(x)    IM_ASSERT(x)
 #define STBRP_STATIC
 #define STB_RECT_PACK_IMPLEMENTATION
 #include "imstb_rectpack.h"
+#endif
 
 struct ImFontBuildSrcGlyphFT
 {