Progressive image read support

[randy408]

The code is already structured to handle row callbacks, this depends on #10, the testsuite also needs some refactoring before tests can be added for this.

[randy408]

Progressive image decoding will be implemented without callbacks, chunk/row callbacks are now tracked by #40.

[randy408]

The current API is as follows:

struct spng_row_info
{
    uint32_t scanline_idx, row_num;
    int pass;
    uint8_t filter;
};

int spng_decode_scanline(struct spng_ctx *ctx, void *out, size_t size);
int spng_decode_row(struct spng_ctx *ctx, void *out, size_t size);
int spng_get_row_info(struct spng_ctx *ctx, struct spng_row_info *row_info);

spng_decode_scanline() decodes a single scanline, while spng_decode_row() decodes a scanline and deinterlaces it, when the image is not interlaced their behavior is identical.

spng_get_row_info() copies row information for the to-be-decoded scanline to row_info.

struct spng_row_info exposes values that change between spng_decode_row/scanline() calls.
row_num is the row number in the deinterlaced image,
scanline_idx is the scanline index within the current subimage.
pass is the pass number, filter is the scanline's filter byte.

Example usage:

struct spng_row_info row_info;
size_t out_width = image_size / ihdr.height;
size_t offs;

do
{
   if(spng_get_row_info(ctx, &row_info)) goto fail;

   offs = row_info->row_num * out_width;

   ret = spng_decode_scanline(ctx, buf + offs, out_width);
}while(!ret)

if(ret != SPNG_EOI) goto fail;

Row callbacks (not yet implemented) would differ from libpng's, spng_decode_scanline/row() has to be called inside the function and would probably look something like this:

int spng_row_cb(struct spng_ctx *ctx, const struct spng_row_info *row_info);

With libpng the decoded scanline/row has to be copied from an internal buffer before returning from the callback, libspng would decode to a user-supplied buffer directly.