create struct screenDisplayBuffer type (cellDisplayBuffer[COLS][ROWS] ---> screenDisplayBuffer)

[Nathan-Fenner]

Before this change, most of the time, a cellDisplayBuffer buf[COLS][ROWS] was created wherever needed. Now, you can spell this as screenDisplayBuffer buf;

There are a few ways that this is nicer:

• The new type remembers the correct size for the buffer, so it doesn't have to be written down [COLS][ROWS] every time
• The new type is a struct instead of an array, so it has more-consistent semantics: no pointer decay; taking references is explicit instead of implicit, etc.

This PR just swaps out all of the existing cellDisplayBuffer arrays to references to this new type. There's no other changes.

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Eventually, I'd like to refactor a bit more, so that the screen is more "encapsulated" - references to .cells should be restricted to a handful of functions, so that the screen buffer can become a higher-level data structure (e.g. we could eventually store info about "layers" or "effects" to make the view prettier on platforms that support it). Having a consistent "screen" type is an important basic step to make that happen.

[flend]

Makes sense to me.