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
.