Correct the calibration when the axes are inverted.

I had a problem to calibrate a touch screen with the axes inverted. Looking at the code I found that when the axes inversion is detected, the {x,y}_{max,min} are swapped:

    // calculate average of clicks
    float x_min = (clicked.x[UL] + clicked.x[LL])/2.0;
    float x_max = (clicked.x[UR] + clicked.x[LR])/2.0;
    float y_min = (clicked.y[UL] + clicked.y[UR])/2.0;
    float y_max = (clicked.y[LL] + clicked.y[LR])/2.0;

    // Should x and y be swapped?
    if (abs(clicked.x[UL] - clicked.x[UR]) < abs(clicked.y[UL] - clicked.y[UR])) {
        std::swap(x_min, y_min);
        std::swap(x_max, y_max);

However I think that when the axis are swapped the calculation of the
{x,y}_{max,min} are wrong.
Let me explain why:

During the calibration, on the screen are showed four crosses called UL, LR, LL, UR (Upper Left, Lower Right...).

If the axes are not swapped during the calibration we got the following coordinates (x,y the numbers are hypothetical):

    UL -> 10,10            UR -> 90,10
    LL -> 10,70            LR -> 90,70

and

    x_min = (10+10)/2 = 10
    x_max = (90+90)/2 = 90
    y_min = (10+10/2 = 10
    y_max = (70+70)/2 = 70

Instead if the axes are inverted

    UL -> 10,10            UR -> 10,90
    LL -> 70,10            LR -> 70,90

and

    x_min = (10+70)/2 = 35
    x_max = (10+70)/2 = 35                   x_min == x_max
    y_min = (10+90)/2 = 45
    y_max = (10+90)/2 = 45                   y_min == y_max

which are basically wrong. I think that the right solution should be to calculate the
{x,y}_{max,min} as following when an axis inversion is detected:

        x_min = (clicked.x[UL] + clicked.x[UR])/2.0;
        x_max = (clicked.x[LL] + clicked.x[LR])/2.0;
        y_min = (clicked.y[UL] + clicked.y[LL])/2.0;
        y_max = (clicked.y[UR] + clicked.y[LR])/2.0;

( Note the different array indexes )
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()) to
detect the size of the current screen, which is called both in the
GuiCalibratorX11() constructor and in the readrw() method.