View Navigation: using delta mouse movement (instead of force mouse warping)

@ideasman42 this is supposed to fix the things for linux. However the result is slightly different from what I get in OSX with the OPTYPE_CURSOR_WRAP.

It's subtle but opening a Blender build side -by-side of each branch (fps and fps-cursor-warp) shows that fps (with OPTYPE_CURSOR_WRAP) is more smooth.