Terminate drawings immediately on a \p0 tag

Terminate drawings immediately on a \p0 tag

The parsing code allowed override tags after \p0 to affect the drawing.
This is incorrect. Finish the drawing object as soon as \p0 is
encountered instead.

This requires moving the code executing the style overrides from
get_next_char() in ass_parse.c to the main render loop in
ass_renderer.c, because we have to re-enter the rendering loop
inside of a tag. The old code was simply executing all tags until
a new character could be returned to the renderer loop, mutating up
the state (RenderContext fields) for the drawing after the drawing
was closed, but before it was rendered.

This fixes libass issue #47.