Fix typo in GD stub

Generate functions entries from stubs for another set of extensions

Closes GH-5351

Verify that all stubs have a return type

Add imagegetinterpolation()

While `imagesetinterpolation()` is available as of PHP 5.5.0,
there is no according getter function, so users would have to track the
current interpolatio

Add imagegetinterpolation()

While `imagesetinterpolation()` is available as of PHP 5.5.0,
there is no according getter function, so users would have to track the
current interpolation method manually.

To remedy this, we introduce `imagegetinterpolation()` as thin wrapper
for `gdImageGetInterpolationMethod()` (which has been introduced with
libgd 2.1.1), and use `im->interpolation_id` as fallback for older
libgd. Since our bundled libgd does not yet have this function, we add
it.

We also simplify the recently introduced bug79068.phpt, where it is
sufficient to check that the interpolation method has not been changed.

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Fix smaller issues with stubs

GH-5025

Add union return types with one class

Make the $num_points parameter of php_imagepolygon optional

That parameter is mostly useless in practise, and likely has been
directly ported from the underlying `gdImagePolygon()` and f

Make the $num_points parameter of php_imagepolygon optional

That parameter is mostly useless in practise, and likely has been
directly ported from the underlying `gdImagePolygon()` and friends,
which require that parameter since the number of elements of the point
array would otherwise be unknown. Typical usages of `imagepolygon()`,
`imageopenpolygon()` and `imagefilledpolygon()` pass `count($points)/2`
or hard-code this value as literal. Since explicitly specifying this
parameter is annoying and error-prone, we offer the possibility to omit
it, in which case the `$points` array must have an even number of
elements, and the number of points is calculated as `count($points)/2`.

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Add union return types for function stubs

Convert GD Resources to objects

Fix arginfo

The second parameter of `imagexbm()` is nullable.

Fix arginfo

Add GD function stubs