WindowServer: Coalesce flushing buffers into one ioctl() call

We regularily need to flush many rectangles, so instead of making many
expensive ioctl() calls to the framebuffer driver, collect the
rectangles and only make one call. And if we have too many rectangles
then it may be cheaper to just update the entire region, in which case
we simply convert them all into a union and just flush that one
rectangle instead.

Kernel/Graphics/FramebufferDevice.cpp

@@ -215,6 +215,8 @@ int FramebufferDevice::ioctl(FileDescription&, unsigned request, FlatPtr arg)
             return -EFAULT;
         return 0;
     }
+    case FB_IOCTL_FLUSH_BUFFERS:
+        return -ENOTSUP;
     default:
         return -EINVAL;
     };