wrong color-space metadata after encode from RGB

this refers to CESNET/UltraGrid#341

hi, when QSV is compressing from a RGB format (BGRA for now; but applies also to X2RGB), it converts unconditionally to limited-range YUV BT.601, but sets stream metadata according to AVCodecContext::color_primaries and AVCodecContext::color_range. If those are not set explicitly, it sets **full-range** to metadata, anyways.

Please see following test case:
1. input
`ffmpeg -y -f lavfi -i color=#ff0000 -t 1 -pix_fmt bgra [params] -c:v hevc_qsv out.h265`

3. output evaluation

mediainfo --Language=raw --Full --Inform='Video;%matrix_coefficients%,%colour_range%' out.h265 &&
ffmpeg -y -loglevel quiet -i out.h265 -strict -1 -frames 1 out.y4m; sed '1,2d' < out.y4m | od -t x1

input RGB=[0xff,0x00,0x00] corresponds¹ to **full-range** YCbCr BT.601 [76,85,255]=**[0x4c,0x55,0xff]**, **limited** range [65,91,237]=**[0x51,0x5b,0xed]** ²

¹ computed using [this coefficient](https://gist.github.com/yohhoy/dafa5a47dade85d8b40625261af3776a); Cb,Cr coefficients are offset by 128
² or Y=67=0x53 if using Y range 16-240, which I think is correct

evaluation:
1. no params for 1. above - [0x51,0x5b,0xef], mediainfo: ",Full"
2. `-colorspace bt709` - [0x51,0x5b,0xef], mediainfo: "BT.709,Full"
3. `-colorspace bt709 -color_range tv`- (ditto), mediainfo: "BT.709,Limited"
4. `-color_range tv` - (ditto), mediainfo: ",Limited"
5. `-colorspace smpte170m -color_range tv` - (ditto), mediainfo: "BT.601,Limited"

So, in all cases, the output is in limitted-range YCbCr 601, but metadata are correct only in case 5 (and also 4, assuming that implicit value is OK).

PS: `-vf lavfi -i color=#ff0000 -t 1 -pix_fmt bgra` seems to convert to [0x00,0x00,0xfd,0xff] - the swscale conversion is not entirely accurate but I don't know how to produce single-color RGB directly, without conversion from YUV (`-vf lavfi -i color` produces YUV)