Fix and optimize yuv blend in the drawtext filter

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

libavfilter/vf_drawtext.c

@@ -371,12 +371,14 @@ static int config_input(AVFilterLink *inlink)
 
 #define SET_PIXEL_YUV(picref, yuva_color, val, x, y, hsub, vsub) {           \
     luma_pos    = ((x)          ) + ((y)          ) * picref->linesize[0]; \
-    chroma_pos1 = ((x) >> (hsub)) + ((y) >> (vsub)) * picref->linesize[1]; \
-    chroma_pos2 = ((x) >> (hsub)) + ((y) >> (vsub)) * picref->linesize[2]; \
     alpha = yuva_color[3] * (val) * 129;                               \
     picref->data[0][luma_pos]    = (alpha * yuva_color[0] + (255*255*129 - alpha) * picref->data[0][luma_pos]   ) >> 23; \
-    picref->data[1][chroma_pos1] = (alpha * yuva_color[1] + (255*255*129 - alpha) * picref->data[1][chroma_pos1]) >> 23; \
-    picref->data[2][chroma_pos2] = (alpha * yuva_color[2] + (255*255*129 - alpha) * picref->data[2][chroma_pos2]) >> 23; \
+    if(((x) & ((1<<(hsub))-1))==0 && ((y) & ((1<<(vsub))-1))==0){\
+        chroma_pos1 = ((x) >> (hsub)) + ((y) >> (vsub)) * picref->linesize[1]; \
+        chroma_pos2 = ((x) >> (hsub)) + ((y) >> (vsub)) * picref->linesize[2]; \
+        picref->data[1][chroma_pos1] = (alpha * yuva_color[1] + (255*255*129 - alpha) * picref->data[1][chroma_pos1]) >> 23; \
+        picref->data[2][chroma_pos2] = (alpha * yuva_color[2] + (255*255*129 - alpha) * picref->data[2][chroma_pos2]) >> 23; \
+    }\
 }
 
 static inline int draw_glyph_yuv(AVFilterBufferRef *picref, FT_Bitmap *bitmap, unsigned int x,