When drawing text, mark the surface as dirty.

When drawing text, mark the surface as dirty.

Since libi3 currently creates its own cairo surface for drawing text, we
need to mark our own surface as dirty to force cairo to invalidate its
cache. Otherwise, this will result in graphical glitches such as the text
not showing up at all.

This wrapper can be removed in the future when libi3 is adapted to reuse
the same cairo surface as we do for all other drawing operations.