ofEasyCam: fixes floating point errors in rotation + events priority

Fixes this problem: http://forum.openframeworks.cc/t/ofeasycam-roll-when-dragging-problem/14424/6  which was caused from calculating the rotation from the difference with the last mouse position which introduced small floating point errors on every update. Calculating the rotation from the difference of the mouse position when it's pressed and the current removes the precision errors and fixes the problem.

Also starts the movement on the mouse events so other code like guis can intercept the event and the camera won't move while moving a slider for example