videomixer: I420 blending : Fix main algorithm.

When blending a source layer with an alpha of 'a' on top of another
destination layer we take the sum of:
* 'a' percent of the source layer
* (100 - 'a') percent of the destination layer (the remainder)

gst/videomixer/blend_i420.c

@@ -192,7 +192,7 @@ gst_i420_do_blend (guint8 * src, guint8 * dest,
   int i, j;
   for (i = 0; i < src_height; i++) {
     for (j = 0; j < src_width; j++) {
-      *dest = src_alpha * (*dest) + (1. - src_alpha) * (*src);
+      *dest = src_alpha * (*src) + (1. - src_alpha) * (*dest);
       dest++;
       src++;
     }