For text output, allocate exactly the right buffer size.

For JSON output, return error codes as an array, not just a
concatinated text string.

Write column data into a buffer, and only write
to USB once the buffer is filled past threshould.

This reduces the amount of data frames on the USB bus,
in a 429x336 sample image it was reduced from 1000 frames
to about 100 frames, where the image data is only 20 frames,
while the rest is mainly polling for status updates.

Access to ctx is needed for the next patch

Increase sleep from 50 μs to 5 ms.

Compiling gives the following error, since the library is specified
before the local objects:

$ make
[…]
cc -lpng16  build/main.o build/ql.o build/loadpng.o -o build/qlprint
/usr/bin/ld: build/loadpng.o: in function `loadpng':
…/src/loadpng.c:30: undefined reference to `png_sig_cmp'
/usr/bin/ld: …/src/loadpng.c:35: undefined reference to `png_create_read_struct'
/usr/bin/ld: …/src/loadpng.c:39: undefined reference to `png_create_info_struct'
/usr/bin/ld: …/src/loadpng.c:43: undefined reference to `png_create_info_struct'
[…]
collect2: error: ld returned 1 exit status
make: *** [Makefile:28: build/qlprint] Error 1

After reordering:

$ make
[…]
cc build/main.o build/ql.o build/loadpng.o -lpng16  -o build/qlprint