vector layer: performance measurement pipeline

[adam-ce]

In order to properly test performance, we need several things:

• use shader preprocessor defines from commit 227bbd6 to forward constants to shaders for faster switching.
• implement an automatic benchmark run, that covers several camera positions, waits until the tiles are loaded (for instance by checking the scheduler stats from commit c8255ab) and measures the average fps for 1 second. at the end, a csv snippet should be produced with date time, camera position, fps. i think this would be easiest to do with javascript in qml.