Fix cairo mixers second image bleeding.

The second image would appear as the background into tranparent areas of
the first image.
See MLT bug: https://sourceforge.net/p/mlt/bugs/234/

include/frei0r_cairo.h

@@ -225,16 +225,18 @@ double frei0r_cairo_get_scale (double norm_scale)
  *
  * \param rgba the image buffer with format F0R_COLOR_MODEL_RGBA8888
  * \param pixels the size of the image buffer in number of pixels
+ * \param alpha if >= 0, the alpha channel will be set to this value
  * \see frei0r_cairo_unpremultiply_rgba
  */
-void frei0r_cairo_premultiply_rgba (unsigned char *rgba, int pixels)
+void frei0r_cairo_premultiply_rgba (unsigned char *rgba, int pixels, int alpha)
 {
   int i = pixels + 1;
   while ( --i ) {
     register unsigned char a = rgba[3];
     rgba[0] = ( rgba[0] * a ) >> 8;
     rgba[1] = ( rgba[1] * a ) >> 8;
     rgba[2] = ( rgba[2] * a ) >> 8;
+    if (alpha >= 0) rgba[3] = alpha;
     rgba += 4;
   }
 }

src/mixer2/cairoaffineblend/cairoaffineblend.c

@@ -303,8 +303,8 @@ void f0r_update2(f0r_instance_t instance, double time, const uint32_t* inframe1,
   unsigned char* out = (unsigned char*)outframe;
   int pixels = inst->width * inst->height;
 
-  frei0r_cairo_premultiply_rgba (src, pixels);
-  frei0r_cairo_premultiply_rgba (dst, pixels);
+  frei0r_cairo_premultiply_rgba (src, pixels, 0xff);
+  frei0r_cairo_premultiply_rgba (dst, pixels, -1);
   draw_composite (inst, out, src, dst, time);
   frei0r_cairo_unpremultiply_rgba (out, pixels);
 }

src/mixer2/cairoblend/cairoblend.c

@@ -185,8 +185,8 @@ void f0r_update2(f0r_instance_t instance, double time, const uint32_t* inframe1,
   unsigned char* out = (unsigned char*)outframe;
   int pixels = inst->width * inst->height;
 
-  frei0r_cairo_premultiply_rgba (src, pixels);
-  frei0r_cairo_premultiply_rgba (dst, pixels);
+  frei0r_cairo_premultiply_rgba (src, pixels, 0xff);
+  frei0r_cairo_premultiply_rgba (dst, pixels, -1);
   draw_composite (inst, out, src, dst, time);
   frei0r_cairo_unpremultiply_rgba (out, pixels);
 }