Correct screen size detection

The XRandR api was used in a such way to get the *DEFAULT* size instead the
corrent one. This patch create a helper function (detect_display_size()) which
detects the current size of the screen; it  is called both in the
GuiCalibratorX11() constructor and in the redraw() method.
Merge pull request #51 from kreijack/screen_size

Correct screen size detection