Version 1.2.1

- added support for GDEW075T7 7.5" b/w 800x480
- GDEW075T7 has differential update (1.6s) using a charge balancing waveform
- added optional SW SPI support, see /extras/sw_spi/README
- added /extras/tests/GxEPD2_RefreshTests/GxEPD2_RefreshTests.ino, for waveform tuning
- minor fixes
- note that 7.5" e-paper displays don't work reliable if fed from 3.3V Arduino pin