opengl: simplify cm pipeline

fixes a few mistakes, and skips the CM shader in cpu instead of adding a costly branch

ref #9641

src/render/Shader.hpp

@@ -12,7 +12,6 @@ class CShader {
     GLint   color             = -1;
     GLint   alphaMatte        = -1;
     GLint   texType           = -1;
-    GLint   skipCM            = -1;
     GLint   sourceTF          = -1;
     GLint   targetTF          = -1;
     GLint   sourcePrimaries   = -1;