lavu/eval: add clip function
lavfi/drawtext: add expansion function eif()

It evaluates expression and outputs it as integer value, using specified
format.

Address trac ticket #3699.

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
lavfi/drawtext: add fontcolor_expr option

Allow to dynamically evaluate the font color.

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
doc/filters/drawtext: simplify example, use clip() eval function