Firmware: AMOLED panels draw a true black background

[tozes]

What and why

An AMOLED pixel that is off draws no power, but the UI painted its background near black (rgb565(10,14,20)), so every background pixel was lit. DisplayInfo gains emissive; AmoledDisplay sets it (the 1.75, 1.75C and 1.8), and Ui draws the background black on emissive panels. Backlit LCDs keep the near-black background, and the grey bars on rectangular panels stay.

Part of a series for the AMOLED-1.75C; the physical report is #103.

How you know it works

Tests on this branch (based on 8069101):

• hermes-gadget build-sim --test: core 122/122. The new "display: an emissive panel leaves its background unlit; a backlit one keeps near black".
• pytest without the simulator-window tests (no virtual display here): 269 passed, 29 skipped.
• pio run builds: esp32s3-touch-amoled-175c, esp32s3-touch-amoled-18. CI builds every board.

Board: Waveshare ESP32-S3-Touch-AMOLED-1.75C, SKU 33691 (the unit in #103), with Hermes v0.21.6+199.g1744a19 on Fedora 44. The checks ran on builds of a working branch that combined this change with the others in this series (30f8bc0); this branch's own image was not installed on its own.

Physical: the background looked true black on the 1.75C, with nothing else visibly changed.

Documentation and sources

docs/porting.md (what emissive means for a display driver) and a changelog line.

Checklist

• One change per PR, with a title that says what it does
• Relevant docs updated, or the reason no changes apply is explained
• Verification results and remaining limitations recorded
• Applicable vendor sources, licenses, and contributor credit retained
• For a new board: the checklist in CONTRIBUTING.md (not a new board)

[tozes]

Thanks for the quick reviews and the follow-up fixes on #103–#105.

A heads-up on what's coming: #106, #109 and #111 are the next three, and I have more 1.75C work lined up after them (battery indicator, Wi-Fi backoff, battery curves, light sleep, and two plugin changes). Those are open as drafts (#107, #108, #110, #112–#116) so you can see where it's going, but there's no need to look at them yet. I'll mark them ready a few at a time as these merge, so the review load stays manageable. If you'd prefer a different pace or order, or want any of them scoped differently, just lmk.

I'm excited about this project and would love to keep contributing.