backdrop: make width and height customizable

repos/gems/src/app/backdrop/README

@@ -68,6 +68,14 @@ covering the whole screen or as a single image. The supported attributes are
   viewport can be defined via the 'anchor', 'xpos', and 'ypos' attributes.
 
 
+Customized size
+---------------
+
+By default, the backdrop adjusts itself to the physical screen size as
+reported by the nitpicker GUI server. This default size can be overridden by
+specifying a fixed size as '<config>' attributes 'width' and 'height'.
+
+
 Example
 ~~~~~~~
 

repos/gems/src/app/backdrop/main.cc

@@ -53,8 +53,7 @@ struct Backdrop::Main
 	{
 		Nitpicker::Connection &nitpicker;
 
-		/* physical screen size */
+		Framebuffer::Mode const mode;
-		Framebuffer::Mode const mode = nitpicker.mode();
 
 		/**
 		 * Return dataspace capability for virtual framebuffer
@@ -84,8 +83,8 @@ struct Backdrop::Main
 		/**
 		 * Constructor
 		 */
-		Buffer(Genode::Env &env, Nitpicker::Connection &nitpicker)
+		Buffer(Genode::Env &env, Nitpicker::Connection &nitpicker, Framebuffer::Mode mode)
-		:	nitpicker(nitpicker),
+		:	nitpicker(nitpicker), mode(mode),
 			fb_ds(env.rm(), _ds_cap(nitpicker)),
 			surface_ds(env.ram(), env.rm(), surface_num_bytes())
 		{ }
@@ -325,7 +324,13 @@ void Backdrop::Main::_handle_config()
 {
 	_config.update();
 
-	_buffer.construct(_env, _nitpicker);
+	Framebuffer::Mode const phys_mode = _nitpicker.mode();
+	Framebuffer::Mode const
+		mode(_config.xml().attribute_value("width",  (unsigned)phys_mode.width()),
+		     _config.xml().attribute_value("height", (unsigned)phys_mode.height()),
+		     phys_mode.format());
+
+	_buffer.construct(_env, _nitpicker, mode);
 
 	/* clear surface */
 	_apply_fill(Xml_node("<fill color=\"#000000\"/>"));