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

まず、issue に記載されている MMTitlebarAppearsTransparent と transparency の4つの組み合わせを macOS で再現し、その後、設定 blurradius を比較に含めます。各関連する組み合わせで titlebar がウィンドウ本体の透明度およびぼかしの挙動と一致し、継ぎ目なく見える状態になれば完了です。