Avoid allocating a new slice on every call to drawSingleFrame

[dweymouth]

Description:

Preallocate a slice to reuse to avoid memory allocations in a tight loop

Side note - after testing this in Supersonic for awhile it seems like the GC is more inclined to give excess memory back to the OS now since we're not allocating so frequently :)

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 65.078% (+0.005%) from 65.073%
when pulling 36b38b6 on dweymouth:drawsingleframe-allocs
into e250829 on fyne-io:develop.

[Jacalz]

Thanks. That's a good improvement. Just one minor note inline :)