Fix add_text_point

[SamuMazzi]

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name: Fix add_text_point about: Create a pull request to help us improve title: Fix add_text_point

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Description: The add_text_point() function used to throw an exception when called because of a simple error during the parsing of the parameters. That has been fixed, but it also has been changed x_offset and y_offset which have been brought to the same variable offset. Actually these variables didn't have a default value then they used to make throw an exception if not specified.

Also the type notation of the x and y values have been changed according on how they are handled in the code.

Fixes #2108