gles2: Add a debug binding for highlighting shaded content

Pressing mode-shift-space s will cause the fragment shaders to be
recompiled, adding a green tint to all composited content.
diff --git a/src/compositor.h b/src/compositor.h
index 4963cda..a0ccb72 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -358,6 +358,8 @@
 	PFNEGLQUERYWAYLANDBUFFERWL query_buffer;
 	int has_bind_display;
 
+	int has_egl_image_external;
+
 	void (*destroy)(struct weston_compositor *ec);
 	void (*restore)(struct weston_compositor *ec);
 	int (*authenticate)(struct weston_compositor *c, uint32_t id);