Image areas with subtitles are upsampled to 4:4:4 in order to render
the subtitles (makes blending easier). Try not to copy the Y plane on
upsampling. The libswscale API requires this, but this commit works it
around by scaling the chroma planes separately as AV_PIX_FMT_GRAY8. The
Y plane is not touched at all. This is done for 420p8 only, which is the
most commonly needed case. For other formats, the old way is used.
Seems to make ASS rendering faster about 15% in the setup I tested.
fb23c3a8ee