add set_default_font and a small example

[cyrgani]

As a complement to get_default_font, this PR adds set_default_font as well as a small example to demonstrate that it works:

This makes it much easier to set the default font once before the main loop and then use the regular text drawing APIs, rather than having to explicitly use draw_text_ex with the font each time.

[not-fl3]

Thanks for all the PRs! 👍