qml: Improve Interactive Blur performance

[fredldotme]

• Remove most of the visually redundant shaders for color correction.
• Update the blur in a deferred manner, leaving room for the shell to render other QML Items.
• Hide the Stage (!) when fully occluded by the Drawer to avoid overdraw.

[ilyakooo0]

Can confirm that the app drawer background blur is noticeably faster on Pixel 3a.