Avoid needing LD_PRELOAD on reMarkable 2

[bkirwi]

Originally discussed here: #62 (comment)

The basic idea is to issue calls to the /opt/lib/librm2fb_client.so.1 library directly, if it's present. This means libremarkable apps on rm2 would "just work" without needing to use the shim explicitly. (Though it would still need to be installed.)

[LinusCDE]

Here is a working (but terribly slow) poc: https://github.com/LinusCDE/libremarkable/tree/explicit_rm2fb_poc

Anyone can feel to expand on that. Permission is granted to copy and reuse this without any credit.

[bkirwi]

A second approach has been implemented at #78 - implementing the rm2fb client logic in Rust instead of depending on librm2fb_client at all.