csputils: add Panasonic V-Gamut primaries

This is actually not entirely trivial since it involves negative Yxy
coordinates, so the CMM has to be capable of full floating point
operation. Fortunately, LittleCMS is, so we can just blindly implement
it.

video/csputils.h

@@ -64,6 +64,7 @@ enum mp_csp_prim {
     MP_CSP_PRIM_PRO_PHOTO,
     MP_CSP_PRIM_CIE_1931,
     MP_CSP_PRIM_DCI_P3,
+    MP_CSP_PRIM_V_GAMUT,
     MP_CSP_PRIM_COUNT
 };