Disable werror for Dear ImGui
GCC 7.5:
../../subprojects/imgui-1.81/imgui.h:1642:80: warning: assuming signed overflow does not occur when assuming that (X - c) <= X is always true [-Wstrict-overflow]
inline T&           operator[](int i)                   { IM_ASSERT(i >= 0 && i < Size); return Data[i]; }
@@ -218,6 +218,7 @@ util_files = files(
imgui_options = [
'default_library=static',
+ 'werror=false',
# use 'auto_features=disabled' once available: https://github.com/mesonbuild/meson/issues/5320
'dx9=disabled',
'dx10=disabled',
