getPoint() should return return unique value for invalid samples.

[MerwinPaterwin]

getPoint() returns a point with a pressure (TSPoint.z) of 0 if the A/D samples are not consistent. This is ambiguous, as a pressure of 0 means that there is no touch which is invariably not the case. I suggest that if the samples are inconsistent, a pressure of -1 be returned, signifying an invalid result. Please change line #169 of TouchScreen.cpp to z=-1;

Thanks.....