[vesafb] Set "magic" colour to transparent when a background picture is used

Use the magic colour facility to cause the user interface background
to become transparent when we have a background picture.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/interface/pcbios/vesafb.c

 #include <realmode.h>
 #include <ipxe/console.h>
 #include <ipxe/io.h>
+#include <ipxe/ansicol.h>
 #include <ipxe/fbcon.h>
 #include <ipxe/vesafb.h>
 #include <config/console.h>
 	if ( ! vesafb_console.disabled ) {
 		vesafb_fini();
 		bios_console.disabled &= ~CONSOLE_DISABLED_OUTPUT;
+		ansicol_reset_magic();
 	}
 	vesafb_console.disabled = CONSOLE_DISABLED;
 
 	vesafb_console.disabled = 0;
 	bios_console.disabled |= CONSOLE_DISABLED_OUTPUT;
 
+	/* Set magic colour to transparent if we have a background picture */
+	if ( config->pixbuf )
+		ansicol_set_magic_transparent();
+
 	return 0;
 }