* PHP-7.3:
  Fix potential integer overflow detected by oss-fuzz

We port the respective fix from upstream[1].

[1] <https://github.com/libgd/libgd/commit/9ed642764cf0b4585d135eb738812a43265cb2d3>

* PHP-7.3:
  Fix #79676: imagescale adds black border with IMG_BICUBIC

We have to loop over all image pixels to avoid the black border.  This
is also done in external libgd in `_gdScaleOneAxis` and `_gdScalePass`.

* PHP-7.3:
  Fix #79615: Wrong GIF header written in GD GIFEncode

The color resolution is expected in bits 4-6 of the packed fields byte
of the logical screen descriptor (byte 10 of the GIF data stream),
according to the specification[1], section 18.

[1] <https://www.w3.org/Graphics/GIF/spec-gif89a.txt>

* PHP-7.3:
  Fix #79068: gdTransformAffineCopy() changes interpolation method

We port
<https://github.com/libgd/libgd/commit/9088591eae437358ee5b929adf82865e37e3001e>.

* PHP-7.3:
  Fix #79067: gdTransformAffineCopy() may use unitialized values

We port
<https://github.com/libgd/libgd/commit/7a06c1669c563917bc48c464521e3de962ddb4e8>.