framebuffer_session: alpha as Mode attribute

Propagate the want of an alpha channel as attribute of Framebuffer::Mode
instead of passing this property as a separate bool argument.

This clears the way for adding useful accessors for pixel/alpha/input
surfaces to the Mode type as a subsequent step.

Issue #5351

repos/os/include/framebuffer_session/framebuffer_session.h

@@ -34,6 +34,7 @@ namespace Framebuffer {
 	struct Mode
 	{
 		Area area;
+		bool alpha;
 
 		size_t bytes_per_pixel() const { return 4; }