OpenGL: Fix shader compilation failure with shadow_to_opacity and unshaded

[dsnopek]

At the moment, if you make a shader that uses both shadow_to_opacity and unshaded when using the Compatibility renderer, it will fail to compile with the following error:

E 0:00:02:007   _display_error_with_code: SceneShaderGLES3: Fragment shader compilation failed:
ERROR: 0:1817: 'ambient_light' : undeclared identifier
ERROR: 1 compilation errors.  No code generated.
  <C++ Source>  drivers/gles3/shader_gles3.cpp:259 @ _display_error_with_code()

This PR fixes that!

This problem doesn't happen with the Forward+ or Mobile renderers, where the result is full transparency (alpha = 0.0), even though the scene shaders in those renders don't set it explicitly, like in this PR. I don't know if it'd be worth changing Forward+ and Mobile to do it explicitly too?

[Repiteo]

Thanks!