Support for Inky Frame 7.3" (using direct pen into PSRAM framebuffer)

[Gadgetoid]

This changeset brings support for the 800x480 pixel Inky Frame 7.3.

To make this feasible with the very limited RAM in Pico W MicroPython, PicoGraphics is backed by a direct pen framebuffer, authored by @AndrewCapon. This draws into PSRAM. The RAM framebuffer is then copied to the screen right before updating. Since the update cycle is 30 seconds, anyway, we don't have to worry about performance.

Testers should use pimoroni-picow_inky_frame-<commit hash>-micropython.uf2 from GitHub actions artifacts.

TODO

• - Rig a test using PSRAM hardfault handler hook

Notes

⚠️ Taupe ("clear") is not the same colour as the smaller displays, seems to do a yellow/green gradient. Not useful as a colour for dithering, etc?