Supporting <text> tags from SVG files

[Lucas-C]

This is a contribution towards #537

It also including a big refactoring of the text rendering logic,
by introducing a new TextRendererMixin class.
One of the main benefits of this is to reduce the size of fpdf/fpdf.py
and decouple the text rendering logic from FPDF.

Minimal script demonstrating the feature, that works with the current code:

from fpdf import FPDF

pdf = FPDF()
pdf.add_page()
pdf.set_font("Times", size=16)
pdf.image("test/svg/svg_sources/text-samples.svg", w=pdf.epw)
print(pdf.pages[1].contents.decode())
pdf.output("pr_1029.pdf")

[Lucas-C]

Currently this PR only contains a base unit test / SVG test sample file, and some minimal "glue" code.

@gmischler: I think we should wait for your work on TextRegions to be complete before tackling this.

Quoting a comment I made there: https://github.com/py-pdf/fpdf2/blob/svg-text/fpdf/svg.py#L1003:

We should reuse code from the line_break and/or text_region modules to implement this.
We could either:

1. handle text regions in this module (svg), with a dedicated SVGText class.
2. handle text regions in the drawing module, maybe by defining a PaintedPath.text() method.

This second option may be the best approach, as we would benefit from the global transformation performed in SVGObject.transform_to_rect_viewport().

But then, we will have to find a way to have PaintedPath objects from the drawing module to benefit from the text rendering logic implemented in other modules, including the font selection mechanism.

How difficult do you expect this to be?