Patch review request, Amarok::(Time)Slider

[Thomas Lübking]

The patch does

a) avoid unnecessary/doub
le updates/repaints
b) clip the repainting to the necessary area

Rerason:
Every repaint will cause a damage event, thus on OpenGL composited desktops a 
pixmap -> texture conversion (what is expensive and scales with the region)  - 
even if there's no visual update in large areas or at all

c) explicitly close painters (on some compilers/optimizations the stack 
deletion can happen with a large delay, what can lead to unbalanced painting 
states and therefore glitches)

d) pass the sliderknob geometry and position calculation to the SvgHandler 
(where the slider is now painted)
	Before the style was invoked for these calculations what lead at best 
accidental success and would fail if the current widget style sliderhandle 
dimensions differed from the Amarok::Slider assumptions (i.e. e.g. if the knob 
width was >> the slider height) - this leads to "judder" even for small 
differences

e) correct the knobrect calculation by taking the knobs padding (i.e. the half 
width) into account and distributing it over the entire slider width rather 
than having some corner case exceptions (leading to a stick -> jump behaviour)

Regards,
Thomas
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