Slider knobs should scale when rendering while zoomed

Slider knobs should scale when rendering while zoomed
https://bugs.webkit.org/show_bug.cgi?id=149835
<rdar://problem/22897080>

Reviewed by Darin Adler.

Make slider knobs follow suit with the rest of the unscaled form controls
by rendering to an offscreen buffer when the page is zoomed or scaled and
then rendering a scaled version of the offscreen buffer onto the page.

* platform/mac/ThemeMac.mm:
(WebCore::drawCellOrFocusRingIntoRectWithView): Helper function for drawing
    cells and/or focus rings.
(WebCore::ThemeMac::drawCellOrFocusRingWithViewIntoContext): Refactored to
    handle drawing slider knobs as well.
* rendering/RenderThemeMac.mm:
(WebCore::RenderThemeMac::paintSliderThumb): Use scaled rendering when necessary.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@190623 268f45cc-cd09-0410-ab3c-d52691b4dbfc