Adds some helpers that check whether a product of multiple
factors (that need to be non-negative: this is enforced)
summed with another non-negative value overflows when
performed as int. Since stb_image mostly works in ints,
this seems like the safest route. Limits size of images
to 2GB but several of the decoders already enforce this
limit (or even lower ones).

Also adds wrappers for malloc that combine a mul-add-with-
overflow-check with the actual malloc, and return NULL
on failure. Then use them when allocating something that
is the product of multiple factors.

For image formats, also add a top-level "is this too big?"
check that gives a more useful error message; otherwise,
the failed mallocs result in an "out of memory" error.
The idea is that the top-level checks should be the primary
way to catch these bugs (and produce a useful error message).
But a misleading error message is still vastly preferable to
a buffer overflow exploit.

Fixes issues #310, #313, #314, #318. (Verified with the
provided test images)

Along the way, this fixes a previously unnoticed bug in
ldr_to_hdr / hdr_to_ldr (missing NULL check); these functions
are called with the result of an image decoder, so NULLs can
definitely happen.

Another bug noticed along the way is that handling of
interlaced 16-bit PNGs was incorrect. Fixing this (along
with the previous modifications) fixes issue #311.

Yet another bug noticed during this change is that reduce_png
did not check the right pointer during its out of memory
check. Fix that too.

Fixes issue #312.

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.

I claimed that if the most significant bit of a 16bit pixel is set,
it should be opaque (as is suggested by some sources on the internet),
but implemented the opposite.
If implemented "correctly", lots of 16bit TGAs become invisible.. so I
guess 16bit TGAs aren't really supposed to have an alpha-channel, or at
least most 16bit TGAs (despite having set an "alpha-bit" in the "image
descriptor byte") in the wild don't seem to work like that.

So just assume 16bit non-greyscale TGAs are always STBI_rgb without
an alpha channel.

* Calculate correct stb format (incl. proper 16bit support) also when
  using a colormap (palette)
* Create colormap with tga_comp, to correctly support 16bit RGB
  (instead of using tga_palette_bits/8 and just copying the data)
* For TGAs with colormap, the TGA bits per pixel field specifies the
  size of an index to the colormap - the "real" color depth
  of the image is saved in the color map specification's bits per pixel
  field. I think only 8 and 16bit indices make sense (16 should be
  supported, otherwise the colormap length could be u8 instead of u16),
  so I added support for both.
* Helper functions stbi__tga_get_comp() to calculate stb pixelformat and
  stbi__tga_read_rgb16() to read one 16bit pixel and convert it to
  24/32bit RGB(A) - for less duplicate code

* for paletted images, .._info()'s comp should be based on the palette's
  bits per pixel, not the images bits per pixel (which describes the
  size of an index into the palette and is also checked now)
* make sure the color (map) type and the image type fields of the header
  are consistent (=> if TGA color type is 1 for paletted, the TGA image
  type must be 1 or 9)
* .._test() does some more checks and uses stbi__get16le() instead of
  stbi__get16be() - TGA is little endian.
* .._test() now always rewinds (sometimes it used to do only return 0;
  without rewinding)
* remove "error check" at the beginning of stbi__tga_load(), because
  all that is already tested in stbi__tga_test()

stbi__tga_* assumed that 16bit TGAs were Grayscale + Alpha.
However, if the TGA imagetype is not one of the gray ones, it's 16Bit
RGB data, with 5 Bits per channel. If the TGA image descriptor field
has alpha bits (the 3 least significant ones) set, the pixel's most
significant bit is for alpha: 1 for opaque and 0 for translucent.
Furthermore people claim that TGAs can also pretend to have 15bpp,
which is the same as 16bpp but definitely without alpha.

So 15/16bpp TGAs are now decoded to STBI_rgb(_alpha).

This fixes a (well, actually the only) compiler warning.

fix incorrect initialization of alpha channel for RGB PSD

- conversion from int to stbi_uc
- unused parameter

fix bug where we couldn't rewind a file that reached EOF, which can happen with < 92-byte PIC,PNM,HDR,TGA

adjust credits