Added brightness changes

[Jegp]

In recent rm2fb versions, reStream output is too dark to be useful. This PR adds a filter suggested by @D1mme and fixes #91

[psigen]

This isn't my post, but someone made [the following comment in Reddit]:(https://www.reddit.com/r/RemarkableTablet/comments/143gqit/comment/jnaibq3/)

The proper, retro-compatible way to fix it would be something like this (to whoever could be listening and able):

# RM2 only uses the 4 least significant bits (16 shades of gray)
raw_fb_8bit = bytearray()
for i, b in enumerate(raw_fb_4bit):
   raw_fb_8bit.append((b & 0b00001111) * 17)

[rien]

This change is added by #100