SDL implementation references an obsolete field

[nvladimiroff]

It looks like imgui_impl_sdl2.rb references a field of ImguiIO--ClipboardUserData--that was removed in 95c1a84.

This causes the bindings to crash when hitting that line with a 'FFI::Struct#[]=': No such field 'ClipboardUserData' (ArgumentError) error.

My code is a bit of a mess, so getting an isolated example is a bit tricky, but lemme know if that'd help!

[vaiorabbit]

I’m busy recently and don’t have enough time to fix that issue. So please fork this repo and use your own fixed version for now, sorry.