ext/gd: de-factorize image output functions

The image output functions imagegif/imagepng/imagewebp/imageavif/imagejpeg
were all calling the static function _php_image_output_ctx which was basically
a big switch statement between each image type which also have different
parameters.

The only identical part was the call to create_stream_context_from_zval that
is now merged with create_output_context.

Author: thg2k