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

Beginne damit, auf macOS die vier im Issue beschriebenen Kombinationen aus MMTitlebarAppearsTransparent und transparency zu reproduzieren, und beziehe dann die Einstellung blurradius in den Vergleich ein. Erledigt ist die Aufgabe, wenn die titlebar in jeder relevanten Kombination dem Transparenz- und Unschärfeverhalten des Fensterinhalts entspricht und nahtlos erscheint.