1.0 segfaults in igOpenGL3RenderDrawData
Hi, I've tried to no avail to use 1.0, but I get this segfault:

```
C:\Users\kraptor\.nimble\pkgs\nimgl-#1.0\nimgl\imgui\impl_opengl.nim(243) igOpenGL3RenderDrawData
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
```

Offending lines are:

```
        if pcmd.userCallback != nil:
            pcmd.userCallback(cmd_list, pcmd.addr)
```

I checked the userCallbacks, and are populated and pcmd.addr is pointing to ImDrawCmd... so no idea what's going on. If I comment that line, it works but there are lots of mising text, etc.