Fix display character handling

[vintagepc]

There are a few issues with the stock display implementation:

• Colour of text is black ✔️
• No brightness support ✔️
• Custom characters don't work ✔️
• Possible wrap/RAM alignment issue. I think the stock does not map the memory correctly for 20x4 displays. I had to hack this to make it work, I probably missed something when chain-writing across boundaries. (Bootloader flashing display is an example) ✔️

[vintagepc]

I've fixed the alignment/wrapping on the display. Whoever implemented the LCD code did not appear to understand the internal VRAM mapping of these displays.

Also fixed some performance problems that suppressed a ridiculous frequency of display redraws. Now it actually only updates if it is flagged as dirty.

[vintagepc]

I spent some time cleaning up the GL code and display memory mapping. CGRAM is now supported, and the character handling is much reduced in footprint (no textures) and properly sized.

[leptun]

That looks so cool! Well done!

[leptun]

@vintagepc how do you plan to add brightness?

[vintagepc]

I'll fade the background color to black as brightness goes down. Doesn't have to be perfect, just a visual indication so we can test it in sim. :)

[vintagepc]

Brightness has been implemented. (in theory) It's wired to the pin but probably not passing the initial brightness check yet. I think we can close this initial hardware bringup task.

[leptun]

Tie the pin low during detection and it will always pass as Backlight enabled.

[vintagepc]

Yup. Also helps if you have the connection wired the right way around 🤣

[leptun]

Hmmm... what about that last custom character? It is the wrong one. Are these characters generated at runtime or is it just a font file that includes them?
We have 8 “regular” custom characters and 3 other that are used only occasionally. This is one of those instances where (I think) the degree sign is replaced by ✔️

[vintagepc]

They are based on what we write to CG RAM (i.e. I implemented CGRAM support). I think there's still some unimplemented/wrong behaviour with the display somewhere in regards to blanking and screen updates. I haven't dived in as that's less of a roadblock to further activity; but I figured a 95% functional display is pretty critical until we resolve #3 .

Quite possible It's kicking the cursor on an update somewhere and clobbering that character when we write a new one.