enable 32 bit indices in imgui

This may reduce performance, but I was able to get implot to run out of
vertices with a relatively small dataset of a few tens of thousands of
points.

The upstream is working on this:

epezent/implot#41

imgui/imconfig.h

@@ -78,7 +78,7 @@
 // Your renderer back-end will need to support it (most example renderer back-ends 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.
-//#define ImDrawIdx unsigned int
+#define ImDrawIdx unsigned int
 
 //---- Override ImDrawCallback signature (will need to modify renderer back-ends accordingly)
 //struct ImDrawList;

imgui/implot.h

@@ -23,6 +23,7 @@
 // ImPlot v0.2 WIP
 
 #pragma once
+
 #include "imgui.h"
 #include "implotFlags.h"