utils: also align alpha planes up to full byte size

I'm not entirely sure why this was originally disabled - the comment justifies
itself by claiming that the alpha plane does not respect pl_bit_encoding,
but the alpha plane is just a normal plane that is sampled like any other
plane, via the same pl_sample_src mechanism (which includes the bits scaling).

I think this may have been a legacy all the way from mpv, where bit encoding
was handled as part of the 3x3 YUV decoding matrix, but we basically stopped
doing that for the majority of inputs a long time ago.

Actually, maybe we should just kill pl_bit_encoding being part of
pl_color_repr altogether at this point.

Author: haasn

src/include/libplacebo/utils/libav_internal.h

@@ -527,14 +527,6 @@ PL_LIBAV_API int pl_plane_data_from_pixfmt(struct pl_plane_data out_data[4],
     first = true;
     for (int p = 0; p < planes; p++) {
         aligned_data[p] = out_data[p];
-
-        // Planes with only an alpha component should be ignored
-        if (pl_plane_data_num_comps(&aligned_data[p]) == 1 &&
-            aligned_data[p].component_map[0] == PL_CHANNEL_A)
-        {
-            continue;
-        }
-
         if (!pl_plane_data_align(&aligned_data[p], &bits))
             goto misaligned;
 

src/utils/upload.c

@@ -113,12 +113,6 @@ bool pl_plane_data_align(struct pl_plane_data *data, struct pl_bit_encoding *out
         if (!aligned.component_size[i])
             break;
 
-        // Can't meaningfully align alpha channel, so just skip it. This is a
-        // limitation of the fact that `pl_bit_encoding` only applies to the
-        // main color channels, and changing this would be very nontrivial.
-        if (aligned.component_map[i] == PL_CHANNEL_A)
-            continue;
-
         // Color depth is the original component size, before alignment
         SET_TEST(bits.color_depth, aligned.component_size[i]);