fix: use dummy surface to track rectangles

[wash2]

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should fix #434

[jacobgkau]

This fixes #434. Seems to eliminate the flicker/movement on first launch in addition to subsequent launches. Tested various panel configurations (hidden/larger/on different edges), and it behaves with all of them.

Regression testing passed:

Launcher

• Cut text from the launcher, then close it; pasting into an app works
• All windows on all workspaces appear on launch
• Choosing an app on another workspace moves workspaces and focus to that app
• Launching an application works
• Typing text and then removing it will re-show those windows
• Search works for applications and windows
• Search works for COSMIC settings panels
• Open windows are sorted above applications (e.g. "firefox")
• t: executes a command in a terminal
• : executes a command in sh
• = calculates an equation
• Search results are as expected:
  • cal returns LibreOffice Calc first
  • pops returns Popsicle first
  • shop returns the COSMIC Store first