Refine GifExportParams

[n0vad3v]

In response to #359 (comment)

Changes:

1. Added notes on GifExportParams to tell users that different vips version will use different params
2. StripMetadata is removed as libvips does not support this function on GIFs, I've tried pass it in with VIPS_OBJECT(operation), "strip", params->stripMetadata,, the output image remains the same.
3. Passed in bitdepth in early vips version

However this PR is not finished, I've found several problems here.

1. No matter how I change GifExportParams's Dither, Effort or Bitdepth, the output image's sha256sum remains the same
2. In the current set_gifsave_options, only one parameter can be used with passed in multiple parameters, as below

  // See for argument values: https://www.libvips.org/API/current/VipsForeignSave.html#vips-gifsave
  if (params->gifDither > 1 && params->gifDither <= 10) {
    ret = vips_object_set(VIPS_OBJECT(operation), "dither", params->gifDither, NULL);
  }
  if (params->gifEffort >= 1 && params->gifEffort <= 10) {
    ret = vips_object_set(VIPS_OBJECT(operation), "effort", params->gifEffort, NULL);
  }
  if (params->gifBitdepth >= 1 && params->gifBitdepth <= 8) {
      ret = vips_object_set(VIPS_OBJECT(operation), "bitdepth", params->gifBitdepth, NULL);
  }

But, as different parameters will result in same output image, I'm not sure why this is happening.

[coveralls]

coverage: 73.277%. remained the same
when pulling 0ed507f on webp-sh:master
into e348555 on davidbyttow:master.

[tonimelisma]

Hey @n0vad3v, thank you very much for the PR. Do you think you will have time finishing it?

[n0vad3v]

Hi @tonimelisma I think this PR is ready to merge for now, however more testing might be needed on GIF images.

[tonimelisma]

It looks like you've removed the StripMetadata parameter. I'm really sorry but we can't have breaking changes in the API. I know it's not optimal to have poorly working or nonfunctional parameters in the API but we can't break users' software.

[n0vad3v]

Got it, now I'm adding StripMetadata option back and added a note for it.

[tonimelisma]

Thank you sir. Your contributions are always welcome!