qml: Even faster blurred Drawer

[fredldotme]

The BackgroundBlur component used to be busy on redrawing the blur source whenever the source rectangle changed. This turned out to be the main reason for weaker devices to have problems drawing with the blur shader enabled.

Instead of changing the source rect based on the Drawer's drag distance, do the following:

• Make the BackgroundBlur stay in place attached to the left-hand side.
• Use a parent item with clipping enabled to cause the blur effect to not show up outside of the Drawer's boundaries.
• Change the blur rectangle to only reference the area of interest.

Fixes blurred Drawer performance on more devices as tested on the JingPad, as well as the weird jelly effect during drag which was introduced in the last performance changeset.

[maciek134]

Verified on the Nexus 5 and 4, no noticeable lag, I guess the only slower device could be the BQ4.5? Similar CPU with more L1 cache than N4, but much slower GPU.

[maciek134]

CI seems to be failing with Target 'tests/CMakeFiles/xvfballtests.dir/all' not remade because of errors., but no details and no other failed tests.