lavfi/drawtext: get bitmap from glyph in a separate step

This change makes it possible to transform the glyph or get its border before
turning it into a bitmap.

Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavfi/drawtext: add option for drawing border around text

Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>