Adding this getbuffer_region method to better support partial refreshes.

[ChrisMenning]

Issue #362 mentions how attempts at performing a Partial Refresh on the EPD 7.5 v2 tends to display garbage most of the time.

I did not have issues with this using the ESP32 driver, but I also experienced garbage output when trying Partial Refreshes using the Python library epd7in5.py. The standard getbuffer method expects the image to match the full display size, but for partial updates, you often want to update only a region of the screen with an image of arbitrary size. getbuffer_region converts any size image to a 1-bit format and inverts the bits as required by the e-paper hardware, allowing you to generate the correct buffer for just the region you want to update. This makes partial updates possible without needing to manipulate or pad the image to the full display size.