This increases write rates (of gifio from #5490) from about 2.4fps to over
5fps by making more efficient use of the SD card protocol.
Because of details of oofatfs, it usually manages 64 writes in a single
CMD25, then two writes in a different area of the SD card (presumably,
filesystem metadata). I couldn't find where to increase "64" to a higher
number. This may depend on the allocation size of the filesystem.
I tried preallocating too, but oddly it significantly lowered the write
rate.
Any trailing data is committed when the file is close()d, or when
the `sync` method of the SDCard object is called.
f22299d2b3