IMGUI do not find correct path to font file

Latest commit on May 9 (60f962) - “Release - label v4 BETA-6” imgui do not start after build:

`./app/build/gui/imgui/sonic-pi-imgui  

[API] [INFO ] : Initialising Daemon

sonic-pi-imgui: /home/mikael/work/sonic-pi/app/gui/imgui/core/imgui_draw.cpp:1978: ImFont* ImFontAtlas::AddFontFromFileTTF(const char*, float, const ImFontConfig*, const ImWchar*): Assertion `(0) && "Could not load font file!"' failed.`



Most probably is commit 88bdb2 - "ImGui - use install root prefix instead of source tree prefix" the cause.

The APP_INSTALL_ROOT in app/gui/imgui/CMakeLists.txt is using the APP_ROOT as default. Unfortunately this is redefined earlier in the same file 'set(APP_ROOT ${CMAKE_CURRENT_LIST_DIR})' from previously being set in the CMakeLists.txt in app directory.



It works if I define APP_INSTALL_ROOT in app/CMakeLists.txt after (APP_ROOT) define instead.

```

if (NOT APP_INSTALL_ROOT)

    set(APP_INSTALL_ROOT ${APP_ROOT})

endif()

```



And change in app/gui/imgui/CMakeLists.txt from

```

if (APP_INSTALL_ROOT)

    target_compile_definitions(${APP_NAME} PRIVATE APP_INSTALL_ROOT="${APP_INSTALL_ROOT}")

else()

    target_compile_definitions(${APP_NAME} PRIVATE APP_INSTALL_ROOT="${APP_ROOT}")

endif()

```

to just

`target_compile_definitions(${APP_NAME} PRIVATE APP_INSTALL_ROOT="${APP_INSTALL_ROOT}")`



However, when looking in the main.cpp file it assumes knowledge on where the imgui directory is with respect to the app directory (gui/imgui) when finding put the file path. So maybe better to change there instead?

```

-    io.Fonts->AddFontFromFileTTF(APP_INSTALL_ROOT "/gui/imgui/res/Cousine-Regular.ttf", 16.0f * vdpi);

+    io.Fonts->AddFontFromFileTTF(APP_INSTALL_ROOT "/res/Cousine-Regular.ttf", 16.0f * vdpi);

```

and also change in app.cpp

```

-    sonic.spApi->Init(fs::path(APP_INSTALL_ROOT) / "..");

+    sonic.spApi->Init(fs::path(APP_INSTALL_ROOT) / "../../..");

```

I also see that the the Qt build do not override APP_ROOT but uses dedicated name (QTAPP_ROOT).

[]