Incorrect rendering of themed non-client window buttons on maximize/restore

Tested build: 0.4.17-dev-768-g8d44ccd

It was first noticed by me, watching Carl's 0.4.16 release video, then noticed by Splitwirez.

Steps to reproduce:

• Start Live environment and go to desktop
• Apply a non-classic window style, such as Mizu
• Open My Computer window and maximize it
  • Expected: The caption buttons are in the corner of maximized window
  • Observed: They are doubled, both in previous position, and in the corner
• Then restore that maximized window to normal
  • Expected: The caption buttons are in the corner where they are supposed to be
  • Observed: They are not rendered at all

	0.4.17-dev-401-g24e904e	0.4.17-dev-402-gb38db9d
Maximized
Restored

This regression has been introduced in 0.4.17-dev-402-gb38db9d while fixing CORE-5877 and CORE-16672.