Renamed IMGUI_INCLUDE_IMGUI_USER_CPP to IMGUI_INCLUDE_IMGUI_USER_INL (more IDE friendly)
64-bit fixes

`int` values are not the same thing as `size_t` nor `ptrdiff_t`. Convert incorrect uses of `int` to more appropriate types.

Fixes warnings for 64-bit compilations.
Merge pull request #56 from seanmiddleditch/master

64-bit fixes
Added a UserData void *pointer so that the callback functions can access user state

"Just in case a project has adverse reactions to adding globals or
statics in their own code."
Moved UserData
Fixed unlikely buffer overrun in InputCharacters (thanks Daniel Collin)