added flag_ImDrawIdx (#151)

Co-authored-by: Stefan Dresselhaus <stefan@dresselhaus.cloud>

dear-imgui.cabal

@@ -126,6 +126,17 @@ flag use-wchar32
   manual:
     True
 
+flag use-ImDrawIdx32
+  description:
+    Use 32-bit vertex indices (default is 16-bit) is one way to allow large meshes with more than 64K vertices.
+    Your renderer backend will need to support it (most example renderer backends support both 16/32-bit indices).
+    Another way to allow large meshes while keeping 16-bit indices is to handle ImDrawCmd::VtxOffset in your renderer.
+    Read about ImGuiBackendFlags_RendererHasVtxOffset for details.
+  default:
+    True
+  manual:
+    True
+
 common common
   build-depends:
       base
@@ -181,6 +192,10 @@ library
     cxx-options: -DIMGUI_USE_WCHAR32
     cpp-options: -DIMGUI_USE_WCHAR32
 
+  if flag(use-ImDrawIdx32)
+    cxx-options: "-DImDrawIdx=unsigned int"
+    cpp-options: "-DImDrawIdx=unsigned int"
+
   if flag(opengl2)
     exposed-modules:
       DearImGui.OpenGL2