Incorrect type for ImTextureID

ImTextureID was changed to ImU64 (i.e. always 64 bits) since v1.91.4. Before, it was a void*. ImGui.NET generation has ImTextureID map to IntPtr, correct for older versions of ImGui, but not newer versions (including the latest v1.91.6.1 release). For newer versions, it should be a ulong rather.

I fear we need to find another major "imgui to .net" importer because this repo does not seem to very updated anymore and barely is being answered.