Actually use values supplied by -s in webcam.go

[gjedeer]

Before this patch:

./webcam -s 640x480
Available formats:
YUYV 4:2:2
Motion-JPEG
Supported frame sizes for format YUYV 4:2:2
320x240
352x288
424x240
640x360
640x480
800x448
960x540
1280x720
Requesting YUYV 4:2:2 1280x720
Resulting image format: YUYV 4:2:2 1280x720

After this patch:

./webcam -s 640x480
(...)
Resulting image format: YUYV 4:2:2 640x480