fix exporting buffers with 3 planes and VA_EXPORT_SURFACE_SEPARATE_LAYERS

[michaelolbrich]

To get the plane offset, y_cb_offset or y_cr_offset must be multiplied with
the pitch for the 'Y' plane. See for example i965_DeriveImage().

Without this, exporting formats with 3 planes where cb_cr_pitch == width/2
is broken, because the offset of the third plane is calculated incorrectly.

[rmader]

Just wanted to drop that I've been hitting this as well. I've worked around it with a dirty Gstreamer patch which usually gives the right results, but not always. Would be nice to still see this land!

[rmader]

P.S.: I verified that this MR fixes playback with Gstreamer 1.24 vampeg2dec on my Thinkpad T400 (GM45/P8400) with the following samples:

• https://kodi.wiki/view/Samples : MPEG2 1080i/29.97 (U.S. ATSC OTA capture w/DD5.1 audio, muxed into MKV)
• https://docs.evostream.com/2.0/userguide_samplecontent.html bun33s.ts