vo_opengl: Ignore approx-gamma for BT.2020-CL

The details of the non-linear transformation from/to BT.2020's constant
luminance system don't really make sense with any other gamma curve,
since changing the gamma curve completely breaks the chroma channels.

video/out/gl_video.c

@@ -936,8 +936,8 @@ static void compile_shaders(struct gl_video *p)
 
     // Linear light scaling is only enabled when either color correction
     // option (3dlut or srgb) is enabled, otherwise scaling is done in the
-    // source space. We also need to linearize for constant luminance systems.
-    if ((!p->is_linear_rgb && use_cms) || use_const_luma) {
+    // source space.
+    if (!p->is_linear_rgb && use_cms) {
         // We just use the color level range to distinguish between PC
         // content like images, which are most likely sRGB, and TV content
         // like movies, which are most likely BT.2020
@@ -952,6 +952,13 @@ static void compile_shaders(struct gl_video *p)
         }
     }
 
+    // We also need to linearize for the constant luminance system. This
+    // transformation really makes no sense with anything other than the
+    // official gamma curves, though. This overrides approx-gamma.
+    if (use_const_luma) {
+        gamma_fun = MP_CSP_TRC_BT_2020_EXACT;
+    }
+
     // Figure out the right color spaces we need to convert, if any
     enum mp_csp_prim prim_src = p->image_params.primaries, prim_dest;
     if (use_cms) {