Support custom stb_rect_pack filename in Freetype (#3062)

Copies the #define magic from imgui_draw.cpp to the imgui_freetype implementation to allow the use of a custom stb rect_pack here as well.
References: fe5347ef94

docs/CHANGELOG.txt

@@ -57,6 +57,8 @@ Other Changes:
 - Window: Fixed a bug with child window inheriting ItemFlags from their parent when the child
   window also manipulate the ItemFlags stack. (#3024) [@Stanbroek]
 - Font: Fixed non-ASCII space occasionally creating unnecessary empty polygons.
+- Misc, Freetype: Fixed support for IMGUI_STB_RECT_PACK_FILENAME compile time directive
+  in imgui_freetype.cpp (matching support in the regular code path). (#3062) [@DonKult]
 - Demo: Added black and white and color gradients to Demo>Examples>Custom Rendering.
 - CI: Added more tests on the continuous-integration server: extra warnings for Clang/GCC, building
   SDL+Metal example, and more compile-time imconfig.h settings: disabling obsolete functions,

misc/freetype/imgui_freetype.cpp

@@ -279,11 +279,17 @@ namespace
 }
 
 #ifndef STB_RECT_PACK_IMPLEMENTATION                        // in case the user already have an implementation in the _same_ compilation unit (e.g. unity builds)
+#ifndef IMGUI_DISABLE_STB_RECT_PACK_IMPLEMENTATION
 #define STBRP_ASSERT(x)     IM_ASSERT(x)
 #define STBRP_STATIC
 #define STB_RECT_PACK_IMPLEMENTATION
+#endif
+#ifdef IMGUI_STB_RECT_PACK_FILENAME
+#include IMGUI_STB_RECT_PACK_FILENAME
+#else
 #include "imstb_rectpack.h"
 #endif
+#endif
 
 struct ImFontBuildSrcGlyphFT
 {