Add ImGuiIO::AddInputCharactersUTF8(char* utf8str)

It'll convert the utf8 string to ImWchar's and passes each of them
to AddInputCharacter().

Very handy for SDL2 SDL_TEXTINPUT events, which provide a buffer with an
UTF-8 string.

imgui.cpp

@@ -778,6 +778,18 @@ void ImGuiIO::AddInputCharacter(ImWchar c)
     }
 }
 
+void ImGuiIO::AddInputCharactersUTF8(const char* utf8chars)
+{
+    // we can't pass more wchars than ImGuiIO::InputCharacters[] can hold so don't convert more
+    static const int wcharBufLen = sizeof(ImGuiIO::InputCharacters)/sizeof(ImWchar);
+    ImWchar wchars[wcharBufLen];
+    ImTextStrFromUtf8(wchars, wcharBufLen, utf8chars, NULL);
+    for(int i=0; i<wcharBufLen && wchars[i] != 0; ++i)
+    {
+        AddInputCharacter(wchars[i]);
+    }
+}
+
 // Math bits
 // We are keeping those static in the .cpp file so as not to leak them outside, in the case the user has implicit cast operators between ImVec2 and its own types.
 static inline ImVec2 operator*(const ImVec2& lhs, const float rhs)              { return ImVec2(lhs.x*rhs, lhs.y*rhs); }