text area transparency and titlebar transparency don't mix well

[edouardp]

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