Reuse calculated background color and do not double-buffer when using SkijaGC

[HeikoKlare]

• Pass background color to SkijaGC if already calculated
• Only use double-buffering on Windows when not using SkijaGC
• Reenable toggling of SkijaGC in ControlExample

[github-actions[bot]]

Test Results

34 tests  +34   31 ✅ +31   2s ⏱️ +2s 60 suites +40    0 💤 ± 0  60 files   +40    1 ❌ + 1   2 🔥 +2 

For more details on these failures and errors, see this check.

Results for commit fbe7438c. ± Comparison against base commit c6ad71d0.