[Enhancement] Live preview speedup

[kdmukai]

Due do slower rendering times in SeedSigner OS (full-screen updates are noticeably slower), we need to optimize the ScanScreen live preview as much as possible. Current dev branch is only rendering the live preview at about 1.6 fps for the live display and 3.1 fps for the QR decoder in SeedSigner OS (see my benchmarking branch to get an onscreen fps display).

In my testing, this PR gets a Pi Zero running SeedSigner OS up to about 3.3 fps for the live preview and a coincidental 3.3 fps for the QR decoder.

On Raspi OS, I've seen this same code range from 4.6fps display / 3.8 fps decode to 3.5 fps display / 4.6 fps decode.

[jdlcdl]

On my pi2...

Baseline w/o this pr.
RaspiOS: 4.5 fps display / 10.9 fps decode
seedsigner-os: 3.2 fps display / 6.5 fps decode

With this pr:
RaspiOS: 8.4 fps display / 9.3 fps decode
seedsigner-os: 6.3 fps display / 6.3 fps decode

Display framerate almost double, decode framerate slightly slower.

All measures above done with your dev_livepreview_benchmarking branch merged in.
At first glance, I did sense that this pr was an improvement, but I wouldn't have guessed it was 2x.
Your benchmarking branch sure would be nice to have, w/ onscreen fps visible, in a way where we could turn it on with a hidden setting, maybe thru a SettingsQR?

[newtonick]

ACK Tested

I did some large PSBT scans comparing decoding performance and did not notice any difference.