vo_opengl: implement the Panasonic V-Log function

User request and not that hard. Closes #3157.

Note that FFmpeg doesn't support this and there's no signalling in HEVC
etc., so the only way users can access it is by using vf_format
manually.

Mind: This encoding uses full range values, not TV range.

video/csputils.h

@@ -81,6 +81,7 @@ enum mp_csp_trc {
     MP_CSP_TRC_PRO_PHOTO,
     MP_CSP_TRC_SMPTE_ST2084,
     MP_CSP_TRC_ARIB_STD_B67,
+    MP_CSP_TRC_V_LOG,
     MP_CSP_TRC_COUNT
 };