When gl_FragDepth is defined in the fragment shader and the depth isn't
written to the driver would reset the value to 0. This PR fixes this by
not putting the gl_FragDepth in the shader when it uses the
`depth_unchanged` qualifier or doesn't find any usage in the GLSL.
Pull Request: https://projects.blender.org/blender/blender/pulls/130016
b2e1677b36