Allow setting of CIRCUITPY_DISPLAY_LIMIT in settings.toml

[RetiredWizard]

Set the maximum supported number of displayio displays. This value overrides the CIRCUITPY_DISPLAY_LIMIT compile-time flag.

I do have some questions about this solution.

I'm not sure if this fixes #1760 since Tannewt has said he hopes to implement a better solution in CP 11. I'm also not sure main.c was the right location to insert the call to malloc_display_memory but I don't know displayio well enough to know if there was a better place somewhere in the displayio modules.

So far I've only been testing with the RP2350 feather (DVI display) with a TFT featherwing (SPI display). I'll go ahead and get some more configurations set up for testing but figured I get this posted so others could start reviewing.

[RetiredWizard]

It looks like boards with built-in displays don't like pointer references to the display structures, I was unsuccessful getting a pointer/array combination of variables to work so I think I'll either have to look at the way the boards with builtin displays are accessing the structures or consider leaving the compile time portion of the array in place and just access the dynamically allocated array elements for display indexes > (compile time)CIRCUITPY_DISPLAY_LIMIT.

Fun for another night.....