RFC: video: Update stb_truetype

This was brought in in 2016 and a number of changes have been made since
then. There does not seem to be much change in functionality, but it is
a good idea to update from time to time.

Bring in the latest version:

   5736b15 ("re-add perlin noise again")

Add a few necessary functions, with dummies in some cases. Update the tests
as there are subtle changes in rendering, perhaps not for the better.

Signed-off-by: Simon Glass <sjg@chromium.org>