`size_t` as appropriate)
- BMP: lessen heap memory usage by only `malloc()`ing a row, not
whole file
- GIF: lessen heap memory usage by paging (also simplifies some
function returns); use standard colour char map
- raster: add `raster_malloc()` to fail > 1GB (avoids very large
output files that most systems can't handle; also lessens to
some degree chances of being victim of OOM killer on Linux)
- GUI: printing scale dialog: set maxima on X-dim and resolution
to keep scale <= 200
