[electrum] Better trezor homescreen support

Summary:
This diff improves the support for setting a custom homescreen on trezor devices:

• The pixels are parsed in the expected order (it used to work by accident)
• The image quality is preserved, no more bleeding color
• The TOIG format is supported, so the Safe 3 device homescreen can be set

Test Plan: Change the homescreen on all devices

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17554