clutter/shader-effect: Initialize shader-type properly

The default value of the ClutterShaderEffect:shader-type
property is CLUTTER_FRAGMENT_SHADER. However, because the
struct field is not actually initialized to it, it ends
up assuming the value 0, which is CLUTTER_VERTEX_SHADER.

Properly initialize ClutterShaderEffect's shader_type to
CLUTTER_FRAGMENT_SHADER.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/846

clutter/clutter/clutter-shader-effect.c

@@ -500,6 +500,7 @@ static void
 clutter_shader_effect_init (ClutterShaderEffect *effect)
 {
   effect->priv = clutter_shader_effect_get_instance_private (effect);
+  effect->priv->shader_type = CLUTTER_FRAGMENT_SHADER;
 }
 
 /**