* `force_filter` being < 0 means the original behavior (i.e. figure out
  the best-performing filter per scanline); any other values 0 <= x <= 4 correspond
  to PNG filters (0 = none, 1 = sub, 2 = up, 3 = average, 4 = Paeth).
* `compression_level` being < 0 equals `compression_level` 8 (the previous value).
  The higher this is, the better the compression should be (though it will use
  more memory).

These new parameters are not (yet) exposed for the higher-level API functions.

The builtin stbi_zlib_compress does not compress as well as zlib or
miniz (which is not too surprising as it's <200 LOC), thus PNGs created
by stb_image_write are about 20-50% bigger than PNGs compressed with
libpng.
This change lets the user supply a custom deflate/zlib-style compress
function, which improves compression a lot. This was requested in #113.

Example for zlib:

#include <zlib.h>
unsigned char* compress_for_stbiw(unsigned char *data, int data_len,
                                  int *out_len, int quality)
{
  uLongf bufSize = compressBound(data_len);
  // note that buf will be free'd by stb_image_write.h
  // with STBIW_FREE() (plain free() by default)
  unsigned char* buf = malloc(bufSize);
  if(buf == NULL)  return NULL;
  if(compress2(buf, &bufSize, data, data_len, quality) != Z_OK)
  {
    free(buf);
    return NULL;
  }
  *out_len = bufSize;

  return buf;
}
#define STBIW_ZLIB_COMPRESS compress_for_stbiw
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"
// ...

This allows the user to change the deflate/zlib compress level used for
PNG compression by changing a global variable.

clang says:
error: non-constant-expression cannot be narrowed from type 'int'
      to 'unsigned char' in initializer list [-Wc++11-narrowing]

so I explicitly cast affected stuff to unsigned char.

Some functions were missing that in the definition, others weren't,
all had it in the declarations.

Added mention of JPG and HDR formats at the top of the file

jo_jpeg.cpp is a Public Domain JPEG writer written by Jon Olick in 2012
http://www.jonolick.com/code.html

My changes to jo_jpeg:
* port to plain C89 (+ // comments, as supported by MSVC6)
* support for 2 comp input (Greyscale+Alpha, Alpha is ignored)
* use stbi__write_context abstraction instead of stdio for writing
* adjust names to stbiw-style

tweak license reference wording

stbi_write_hdr_to_func() should still be available even without STBI_WRITE_NO_STDIO,
just like other formats.

Now writing out monochrome bitmaps from 8-bit arrays works
as it does when using PNG.

Bitmaps need 3 bytes per pixel.

Slightly modify the public domain license to keep it in the public domain, but make it clear that even when dedications might not be recognized that the code is still usable.  Given that this isn't dual-licensing under a different license, I'm hoping this will be acceptable.

Slightly modify the public domain license to keep it in the public domain, but make it clear that even when dedications might not be recognized that the code is still usable.  Given that this isn't dual-licensing under a different license, I'm hoping this will be acceptable.

When compiling with more restrictive compiler options
such casting from double to float will cause a warning.

Ex. GCC -Wdouble-promotion
Signed-off-by: NFilip Wasil <filip.wasil@gmail.com>

If calling stbi_write_png concurrently the static array crc_table would be shared between threads causing data-races.

fix internal names to be namspaced properly;

actually want