Use shader include for fore/background shaders

The two shaders use a lot of common logic for co-ordinate and attribute
look up, so it makes sense to use a common include shader for both.

Adds support for the inverse and blink attributes to the foreground shader.

addons/godot_xterm/native/src/terminal.h

@@ -97,6 +97,7 @@ namespace godot
     void initialize_rendering();
     void update_theme();
     void update_sizes(bool force = false);
+    void update_shader_parameters(Ref<ShaderMaterial> material);
     void draw_screen();
     void refresh();
     void cleanup_rendering();