renderer.c: Create new child process for interaction with `cairo`

[personalizedrefrigerator]

Summary

• forks before interaction with cairo in render_frame and render_frame_background
• Cairo interaction is thereby handled by a child process.
• WILL NOT fix most of the crashing bug reports.

Rationale

• Some of the bug reports that report crashes include libcairo in the stack trace.
  • Note that others include xcb, which this will not address.
• Re-launching the program on crash has issues (that may be resolvable):
  • There is a brief interval during which the user's screen is visible
  • During that time, keyboard input isn't captured.
  • See #209