text area transparency and titlebar transparency don't mix well

Issue
The new MMTitlebarAppearsTransparent setting doesn't mix well with the set transparency=<non-zero> setting

MMTitlebarAppearsTransparent=false with set transparency=0

MMTitlebarAppearsTransparent=false with set transparency=<non-zero>

MMTitlebarAppearsTransparent=true with set transparency=0

MMTitlebarAppearsTransparent=true with set transparency=<non-zero>

Additionally, the solution should also work well with the blurradius setting, so the titlebar and window body appear seamless.

To Reproduce
Set MMTitlebarAppearsTransparent as on and transparency to be non-zero.

Expected behavior
The titlebar follows the existing behaviour and appearance of the window body with regard to the transparency and blurradius settings.

Environment (please complete the following information):

• Vim version 8.2.1456
• Macvim version 8.2.1456 (165)
• OS: macOS 10.15.6

Commencez par reproduire sur macOS les quatre combinaisons de MMTitlebarAppearsTransparent et transparency décrites dans l’issue, puis incluez le réglage blurradius dans la comparaison. C’est terminé lorsque la titlebar correspond au comportement de transparence et de flou du corps de la fenêtre et paraît homogène dans chaque combinaison pertinente.