Externals: Use Common ASSERT for IM_ASSERT

This does introduce a dependency on common and on fmt from imgui, but gives a better experience.

Externals/imgui/CMakeLists.txt

@@ -1,3 +1,7 @@
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
 set(SRCS
   imgui.cpp
   imgui_draw.cpp
@@ -8,3 +12,8 @@ set(SRCS
 add_library(imgui STATIC ${SRCS})
 target_include_directories(imgui PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}")
 
+target_link_libraries(imgui
+PRIVATE
+  common
+  fmt::fmt
+)

Externals/imgui/README.txt

@@ -0,0 +1 @@
+When updating, make sure to preserve changes to imconfig.h.  Dolphin modifies it to use a custom assertion handler and to tweak settings.

Externals/imgui/imconfig.h

@@ -14,10 +14,11 @@
 
 #pragma once
 
+#include "Common/Assert.h"
+
 //---- Define assertion handler. Defaults to calling assert().
 // If your macro uses multiple statements, make sure is enclosed in a 'do { .. } while (0)' block so it can be used as a single statement.
-//#define IM_ASSERT(_EXPR)  MyAssert(_EXPR)
-//#define IM_ASSERT(_EXPR)  ((void)(_EXPR))     // Disable asserts
+#define IM_ASSERT(_EXPR)  ASSERT(_EXPR)
 
 //---- Define attributes of all API symbols declarations, e.g. for DLL under Windows
 // Using Dear ImGui via a shared library is not recommended, because of function call overhead and because we don't guarantee backward nor forward ABI compatibility.