sub: better alpha blending when rendering to alpha surfaces

This actually treats destination alpha correctly, and gives much better
results than before. I don't know if this is perfectly correct yet,
though. Slight difference with vo_opengl behavior suggests it might not
be.

Note that this does not affect VOs with true alpha support. vo_opengl
does not use this code at all, and does the alpha calculations in OpenGL
instead.

video/fmt-conversion.c

@@ -49,6 +49,7 @@ static const struct {
     {IMGFMT_BGR4,  AV_PIX_FMT_BGR4},
     {IMGFMT_PAL8,  AV_PIX_FMT_PAL8},
     {IMGFMT_GBRP,  AV_PIX_FMT_GBRP},
+    {IMGFMT_GBRAP, AV_PIX_FMT_GBRAP},
     {IMGFMT_YUYV,  AV_PIX_FMT_YUYV422},
     {IMGFMT_UYVY,  AV_PIX_FMT_UYVY422},
     {IMGFMT_NV12,  AV_PIX_FMT_NV12},