lavfi/vf_psnr: add warning when color ranges differ

The PSNR filter uses the pixel values without considering
the color ranges. This is incorrect. Patch adds a warning
so at least the user knows it.

Let's see an example:

(1) Let's get a simple black pixel/white pixel image.
```
$ echo -n -e "\x00\x00\x00\xff\xff\xff" > /tmp/foo.rgb24
```

(2) From this image, let's distill full and limited range y4m
copies.

```
$ ffmpeg -y -f rawvideo -video_size 2x1 -pix_fmt rgb24 -i /tmp/foo.rgb24 -vf scale="out_range=full" -pix_fmt yuv420p /tmp/foo.full.y4m
$ xxd /tmp/foo.full.y4m
00000000: 5955 5634 4d50 4547 3220 5732 2048 3120  YUV4MPEG2 W2 H1
00000010: 4632 353a 3120 4970 2041 303a 3020 4334  F25:1 Ip A0:0 C4
00000020: 3230 6a70 6567 2058 5953 4353 533d 3432  20jpeg XYSCSS=42
00000030: 304a 5045 4720 5843 4f4c 4f52 5241 4e47  0JPEG XCOLORRANG
00000040: 453d 4655 4c4c 0a46 5241 4d45 0a00 ff80  E=FULL.FRAME....
00000050: 80                                       .
```

and

```
$ ffmpeg -y -f rawvideo -video_size 2x1 -pix_fmt rgb24 -i /tmp/foo.rgb24 -vf scale="out_range=limited" -pix_fmt yuv420p /tmp/foo.limited.y4m
$ xxd /tmp/foo.limited.y4m
00000000: 5955 5634 4d50 4547 3220 5732 2048 3120  YUV4MPEG2 W2 H1
00000010: 4632 353a 3120 4970 2041 303a 3020 4334  F25:1 Ip A0:0 C4
00000020: 3230 6a70 6567 2058 5953 4353 533d 3432  20jpeg XYSCSS=42
00000030: 304a 5045 4720 5843 4f4c 4f52 5241 4e47  0JPEG XCOLORRANG
00000040: 453d 4c49 4d49 5445 440a 4652 414d 450a  E=LIMITED.FRAME.
00000050: 10eb 8080                                ....
```

Note that the 2x images are the same (both have 1x pixel at the
darkest black, and one at the brightest white). Only difference
is the range.

(3) Let's calculate the PSNR score:
```
$ ./ffmpeg -filter_threads 1 -filter_complex_threads 1 -i /tmp/foo.full.y4m -i /tmp/foo.limited.y4m -lavfi "psnr" -f null -
...
[Parsed_psnr_0 @ 0x2f5dac0] PSNR y:22.972065 u:inf v:inf average:25.982365 min:25.982365 max:25.982365
```

As we are comparing an image with itself, we expect "y:inf" as the
luma PSNR. Issue here is that the PSNR filter just uses the pixel
values, ignoring the color ranges.

A possible solution would be to have the filter do the conversion.

Proposed solution is to add a warning.

```
$ ./ffmpeg -filter_threads 1 -filter_complex_threads 1 -i /tmp/foo.full.y4m -i /tmp/foo.limited.y4m -lavfi "psnr" -f null -
...
[Parsed_psnr_0 @ 0x2f5dac0] master and reference frames use different color ranges (pc != tv)
...
[Parsed_psnr_0 @ 0x2f5dac0] PSNR y:22.972065 u:inf v:inf average:25.982365 min:25.982365 max:25.982365
```

Tested:

Ran fate.
```
$ make fate -j
...
TEST    seek-lavf-ppmpipe
TEST    seek-lavf-pgmpipe
TEST    seek-lavf-mxf_opatom
```

libavfilter/vf_psnr.c

@@ -188,6 +188,13 @@ static int do_psnr(FFFrameSync *fs)
         td.planeheight[c] = s->planeheight[c];
     }
 
+    if (master->color_range != ref->color_range) {
+        av_log(ctx, AV_LOG_WARNING, "master and reference "
+               "frames use different color ranges (%s != %s)\n",
+               av_color_range_name(master->color_range),
+               av_color_range_name(ref->color_range));
+    }
+
     ff_filter_execute(ctx, compute_images_mse, &td, NULL,
                       FFMIN(s->planeheight[1], s->nb_threads));