Consider spacing in Convert.text_to_pixels

Currently, Convert.text_to_pixels ignore the spacing specified in the style. This is what we are currently doing: https://github.com/CoffeeStraw/PyonFX/blob/56eee2d1952d4ab60bcc93200909a9d7726cd5d8/pyonfx/font_utility.py#L201-L204 It's the equivalent of these line in VSFilter: https://sourceforge.net/p/guliverkli2/code/HEAD/tree/src/subtitles/RTS.cpp#l288 To properly emulate VSFilter, we would need to add this: https://sourceforge.net/p/guliverkli2/code/HEAD/tree/src/subtitles/RTS.cpp#l276