Use single buffer mode for EInkDisplay (#34)

## Summary

* Frees up 48kB of statically allocated RAM in exchange for 48kB just
when grayscale rendering is needed

## Additional Context

* Upstream changes:
https://github.com/open-x4-epaper/community-sdk/pull/7

platformio.ini

@@ -20,6 +20,7 @@ build_flags =
   -DARDUINO_USB_MODE=1
   -DARDUINO_USB_CDC_ON_BOOT=1
   -DMINIZ_NO_ZLIB_COMPATIBLE_NAMES=1
+  -DEINK_DISPLAY_SINGLE_BUFFER_MODE=1
 # https://libexpat.github.io/doc/api/latest/#XML_GE
   -DXML_GE=0
   -DXML_CONTEXT_BYTES=1024