Incorrect xmpText and langAlt (Xmp) return values from gexiv2_metadata_[try_]get_tag_multiple()

This issue applies to gexiv2 0.12.1.X and others earlier.

When calling gexiv2_metadata_[try_]get_tag_multiple() with an existing xmpText or langAlt type (Xmp) tag, the return value is incorrect.

xmpText:

An xmpText type only contains a single value. If getting a tag (e.g. Xmp.dc.subject) containing the value 1234, the return value would be

array[0] = "1234"
array[1] = "1234"
array[2] = "1234"
array[3] = "1234"
array[4] = nullptr

instead of

array[0] = "1234"
array[1] = nullptr

langAlt:

langAlt types contain language definitions with an optional default value which is always returned in the 0'th position of the array. If getting a tag (e.g. Xmp.dc.title) containing the value "lang=\"x-default\" Monday, lang=\"fr\" Lundi, lang=\"de\" Montag", the return value would be

array[0] = "Monday"
array[1] = "Monday"
array[2] = "Monday"
array[3] = nullptr

instead of

array[0] = "lang=\"x-default\" Monday"
array[1] = "lang=\"fr\" Lundi"
array[2] = "lang=\"de\" Montag"
array[3] = nullptr