Partial window support for 7.5inv2

[badrihippo]

Last month, I managed to get a partial window working on my 7.5" screen (the GDEW075T7) but haven't found the time to iron out everything and clean up the code. I'll get to this by and by, but instead of sitting on it, I decided to paste the code out here in case someone else wants to use it without waiting.

7in5v2partial.snippet.py.txt

This is built upon my previous pull request at #35, as well as the work done later by @colin-nolan in #51. I managed to get a partial window working (only that part of the screen gets refreshed), but not a partial refresh (simple change of pixels with no blink, blink each time). To get it working, you need to

1. append the code to drivers/drivers_partial.py
2. add drivers_partial.EPD7in5v2partial to driverlist (under the get_drivers function in papertty.py)

Like I said the code is very messy, and I'll come clean it up at some point. Just wanted to get the code out in case it helps someone in the meantime.

[joukos]

Thanks for putting it out here! Not sure if I have time to test it but it may help someone else for the time being.