sub: fix --sub-gauss

Implement it directly in sd_lavc.c as well. Blurring requires extending
the size of the sub-images by the blur radius. Since we now want
sub_bitmaps to be packed into a single image, and we don't want to
repack for blurring, we add some extra padding to each sub-bitmap in the
initial packing, and then extend their size later. This relies on the
previous bitmap_packer commit, which always adds the padding in all
cases.

Since blurring is now done on parts of a large bitmap, the data pointers
can become unaligned, depending on their position. To avoid shitty
libswscale printing a dumb warning, allocate an extra image, so that the
blurring pass is done on two newly allocated images. (I don't find this
feature important enough to waste more time on it.)

The previous refactor accidentally broke this feature due to a logic bug
in osd.c. It didn't matter before it happened to break, and doesn't
matter now since the code paths are different.

sub/osd.c

@@ -295,10 +295,6 @@ static void render_object(struct osd_state *osd, struct osd_object *obj,
 
     bool cached = false; // do we have a copy of all the image data?
 
-    if (out_imgs->format == SUBBITMAP_RGBA && opts->sub_gauss != 0.0f)
-        cached |= osd_conv_blur_rgba(obj->cache[2], out_imgs, opts->sub_gauss);
-
-    // Do this conversion last to not trigger gauss blurring for ASS
     if (formats[SUBBITMAP_RGBA] && out_imgs->format == SUBBITMAP_LIBASS)
         cached |= osd_conv_ass_to_rgba(obj->cache[3], out_imgs);