avfilter/vf_mp: preserve pixel format when possible

Fixes Ticket2577
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_mp.c

@@ -249,6 +249,7 @@ typedef struct {
     AVFilterContext *avfctx;
     int frame_returned;
     char *filter;
+    enum AVPixelFormat in_pix_fmt;
 } MPContext;
 
 #define OFFSET(x) offsetof(MPContext, x)
@@ -543,6 +544,10 @@ int ff_vf_next_put_image(struct vf_instance *vf,mp_image_t *mpi, double pts){
     for(i=0; conversion_map[i].fmt && mpi->imgfmt != conversion_map[i].fmt; i++);
     picref->format = conversion_map[i].pix_fmt;
 
+    for(i=0; conversion_map[i].fmt && m->in_pix_fmt != conversion_map[i].pix_fmt; i++);
+    if (mpi->imgfmt == conversion_map[i].fmt)
+        picref->format = conversion_map[i].pix_fmt;
+
     memcpy(picref->linesize, mpi->stride, FFMIN(sizeof(picref->linesize), sizeof(mpi->stride)));
 
     for(i=0; i<4 && mpi->stride[i]; i++){
@@ -796,6 +801,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *inpic)
 
     for(i=0; conversion_map[i].fmt && conversion_map[i].pix_fmt != inlink->format; i++);
     ff_mp_image_setfmt(mpi,conversion_map[i].fmt);
+    m->in_pix_fmt = inlink->format;
 
     memcpy(mpi->planes, inpic->data,     FFMIN(sizeof(inpic->data)    , sizeof(mpi->planes)));
     memcpy(mpi->stride, inpic->linesize, FFMIN(sizeof(inpic->linesize), sizeof(mpi->stride)));