video: support YCgCo colorspace

YCgCo can be manually selected, but will also be used if the decoder
reports YCgCo. To make things more fun, files are sometimes marked
incorrectly, which will display such broken files incorrectly starting
with this commit.

video/csputils.h

@@ -41,6 +41,7 @@ enum mp_csp {
     MP_CSP_SMPTE_240M,
     MP_CSP_RGB,
     MP_CSP_XYZ,
+    MP_CSP_YCGCO,
     MP_CSP_COUNT
 };