video: Add stb TrueType font renderer

This is a header file which provides a fairly light-weight TrueType
rendering implementation. It is pulled from http://nothings.org/

. The code
style does not comply with U-Boot but I think it is best to leave alone to
permit the source to be synced later if needed.

The only change is to fix a reference to fabs() which should route through
a macro to allow U-Boot to provide its own version.

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