avcodec/libvpxenc: fix sRGB colorspace for non-RGB pixel formats

When encoding VP9 with a YUV pixel format (e.g. yuv420p) and
AVCOL_SPC_RGB colorspace metadata, libvpxenc unconditionally set
VPX_CS_SRGB. This produced a spec-violating bitstream: Profile 0
(4:2:0) with sRGB colorspace, which is only valid for Profile 1/3
(4:4:4). The resulting file is undecodable.

Fix this by setting ctx->vpx_cs to VPX_CS_SRGB in set_pix_fmt()
for 4:4:4 YUV formats when AVCOL_SPC_RGB is set, matching the
existing GBRP path. This covers the legitimate case of RGB data in
YUV444 containers (e.g. H.264 High 4:4:4 with identity matrix).

With this change, any AVCOL_SPC_RGB that reaches the switch in
set_colorspace() is guaranteed to be a subsampled format where
sRGB is invalid. Return an error so the user can fix their
pipeline rather than silently producing incorrect output.

To reproduce:

  ffmpeg -f lavfi -i testsrc=s=64x64:d=1:r=1 \
    -c:v libvpx-vp9 -pix_fmt yuv420p -colorspace rgb bad.webm
  ffprobe bad.webm
  # -> "vp9 (Profile 0), none(pc, gbr/...), 64x64"
  ffmpeg -i bad.webm -f null -
  # -> 0 frames decoded, error

See also:
  https://issues.webmproject.org/487307225

Signed-off-by: Guangyu Sun <gsun@roblox.com>
Signed-off-by: James Zern <jzern@google.com>