Add QOI format support

[hovertank3d]

https://github.com/phoboslab/qoi

[sezero]

If @slouken or @icculus wants this, this patch needs changes:

• https://github.com/phoboslab/qoi/blob/master/README.md has scary
  warnings a.out the project's readiness
• qoi.h is not buildable in c90 mode: See the attached patch which
  makes it build in c90 mode.
• None of the libc/libm func redefines in IMG_qoi.c are needed:
  We only need to define QOI_MALLOC and QOI_FREE.
• We don't need qoi.h stdio functionality in IMG_qoi.c: we should
  define QOI_NO_STDIO.
• qoi.h stores ftell result as int but it should be long,
  rest of its api (e.g. qoi_encode) relies on an int size ptr.
  That's minor, but wanted to mention anyway.
• autotools changes are wrong: You should only touch Makefile.am
  and configure.ac, the rest I can handle regenerating here.
• VisualC and Xcode projects need updating for the new addition.

Here are the patches I used:

• patch for IMG_qoi.c: patch-IMG_qoi.txt
• patch for qoi.h: patch-qoi_h.txt
• patch for autotools (excludes generated files): patch-autotools.txt

[sezero]

Additional note: My gcc emits the following warnings:

qoi.h: In function ‘qoi_encode’:
qoi.h:356: warning: missing braces around initializer
qoi.h:356: warning: (near initialization for ‘index[0]’)
qoi.h: In function ‘qoi_decode’:
qoi.h:497: warning: missing braces around initializer
qoi.h:497: warning: (near initialization for ‘index[0]’)

These are qoi_rgba_t index[64] = {0}; and a compiler (e.g. MSVC)
can optimize those to memset(index,0,sizeof(x)) which can be a
problem in SDL's MSVC builds (SDL uses its own SDL_memset proc.)
I suggest that qoi project should expose a QOI_ZERO macro and
use it instead of those initializers, if it does that then SDL can
use its own SDL_zeroa macro for it.

[sezero]

FWIW, I submitted my proposed qoi.h changes to mainstream at phoboslab/qoi#67

[slouken]

@sezero, thanks for the review. The README states 12/20/2021 as the date that the specification will be finalized, so let's wait for that before applying this patch. Hopefully the upstream patches will have landed as well.

[sezero]

And here is a patch to address zero-initializer issue I mentioned above (also submitted to upstream):
patch-zero-initializer.txt

[sezero]

The patches I suggested to qoi mainstream are in, so I updated this PR
to reflect them, along with the build system updates I requested. MSVC
and Xcode project files still need updating, by someone other than me.

@slouken: The rest is up to you.

[sezero]

The format seems to have finalized: phoboslab/qoi#95

I updated the pull request a final time.