imgconvert: favor pixel formats without resolution loss

Fixes Ticket1517
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/imgconvert.c

@@ -543,6 +543,7 @@ enum PixelFormat avcodec_find_best_pix_fmt2(enum PixelFormat dst_pix_fmt1, enum
         ~0, /* no loss first */
         ~FF_LOSS_ALPHA,
         ~FF_LOSS_RESOLUTION,
+        ~FF_LOSS_COLORSPACE,
         ~(FF_LOSS_COLORSPACE | FF_LOSS_RESOLUTION),
         ~FF_LOSS_COLORQUANT,
         ~FF_LOSS_DEPTH,