subpictures: don't use VLC_CODEC_TEXT as a chroma

This is a simplified version of !4445 (closed) where the VLC_CODEC_TEXT is not used as a chroma anymore. And the way to tell a region is text-based is to use the subpicture_region_IsText() macro. Text renderers must call subpicture_region_TextMarkRendered() when a region is successfully rendered into the p_picture.