Fix overflow

[SeanDS]

I had a weird issue with my TJCTM24028-SPI (cheap eBay touch screen + TFT) and this library. When touching the lower 25% of the screen, the XPTPaint program worked ok, but when going into the top 75% the lines jumped off the screen and were not where my finger was touching. I tracked down the cause (or, at least, the change that fixes this) to the calibration variables _cal_vi1 and _cal_vj1. These were previously set to uint16_t, but are used in a calibration operation involving int32_t numbers, potentially leading to overflow (as in my case). Changing these variables to int32_t fixes this overflow, and solves the problem on my screen with discontinuities.