Fix extra 0.5f pixel offset

This was causing the text have the right side of each char cut off in opengl.  Sorry for the messy commit structure ( new to github ), but the idea is simple, add a setting to imgui::imguistyle that has the offset.  default to 0.0f, and change it to 0.5f in <= d3d9, leave 0.0f for ogl & d3d10+.