Use double-precision for rectangle calculations in WVuMeter.

Fixes quantization glitches in SVG rendering.
Unify logic for all Paintable::draw methods into one.

* Remove pixmap-rendering for rendering SVG with a source rectangle. I
  added this hack when I was adding SVG support because I didn't have
  time to figure out the problems it caused for VU meters.
* Adds a FIXED and STRETCH_ASPECT DrawMode to Paintable.
Clean up include orders.
Add Paintable::DrawModeToString.
Add SkinContext::selectScaleMode.
Add Paintable::drawCentered (for WKnobComposed, essentially).
Add scalemode support to most widget images.

* Remove WStatusLight SizeMode as DrawMode is more general and covers
  its use cases.
* Only call QWidget::setFixedSize if the DrawMode is FIXED.
* Use FIXED as default DrawMode for most widgets for backwards
  compatibility.
* Switch most Paintable::draw() calls to rectangle targets instead
  of (x,y) points. This allows the DrawMode to do the right behavior
  based on the mode.
Deere: Set slider stretch options.
Merge pull request #426 from rryan/scaling

Better widget image scaling support.