vout: subpictures: clear borrowed palettes

#7  0x00007fc09f4749b3 in subpicture_region_NewInternal (p_fmt=0x7fc02c3fd950) at ../../src/misc/subpicture.c:227
#8  0x00007fc09f43742a in SpuRenderRegion
    (spu=0x7fbfb4073530, dst_ptr=0x7fbf74000c08, dst_area=0x7fc02c3fdeb0, entry=0x7fbf7403dce0, region=0x7fbf780376d0, scale_size=..., chroma_list=0x7fc02cfdac28 <gl_subpicture_chromas>, fmt=0x7fc02c3feb00, i_original_width=2469, i_original_height=1389, subtitle_area=0x7fc02c3fdf70, subtitle_area_count=0, render_date=193036481405) at ../../src/video_output/vout_subpictures.c:1110
#9  0x00007fc09f437ce1 in SpuRenderSubpictures
    (spu=0x7fbfb4073530, i_subpicture=1, p_entries=0x7fbf7403dce0, chroma_list=0x7fc02cfdac28 <gl_subpicture_chromas>, fmt_dst=0x7fc02c3feb00, fmt_src=0x7fbfb49faa38, system_now=193036481405, render_subtitle_date=193036481405, external_scale=false) at ../../src/video_output/vout_subpictures.c:1239
#10 0x00007fc09f43a621 in spu_Render
    (spu=0x7fbfb4073530, chroma_list=0x7fc02cfdac28 <gl_subpicture_chromas>, fmt_dst=0x7fc02c3feb00, fmt_src=0x7fbfb49faa38, system_now=193036481405, render_subtitle_date=193036481405, ignore_osd=false, external_scale=false) at ../../src/video_output/vout_subpictures.c:1998
#11 0x00007fc09f42c5c4 in PrerenderPicture (sys=0x7fbfb40462a0, filtered=0x7fbfc029b900, render_now=0x7fc02c3febd4, out_pic=0x7fc02c3febf0, out_subpic=0x7fc02c3febf8)
    at ../../src/video_output/video_output.c:1186
#12 0x00007fc09f42c9e3 in RenderPicture (sys=0x7fbfb40462a0, render_now=true) at ../../src/video_output/video_output.c:1281
#13 0x00007fc09f42d469 in DisplayPicture (vout=0x7fbfb40462a0) at ../../src/video_output/video_output.c:1521