Subpixel rendering: Make GraphicsLayer's offsetFromRenderer subpixel position based.

Subpixel rendering: Make GraphicsLayer's offsetFromRenderer subpixel position based.
https://bugs.webkit.org/show_bug.cgi?id=128694

Reviewed by Simon Fraser.

Changing layers from using integral types to using Float/LayoutUnits so that
we can position them on subpixels. They are still integral positioned though.

Covered by existing tests.

* platform/graphics/GraphicsLayer.cpp:
(WebCore::GraphicsLayer::setOffsetFromRenderer):
(WebCore::GraphicsLayer::paintGraphicsLayerContents):
* platform/graphics/GraphicsLayer.h:
(WebCore::GraphicsLayer::offsetFromRenderer): removed incorrect comment.
* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::updateGraphicsLayerGeometry):
(WebCore::RenderLayerBacking::adjustAncestorCompositingBoundsForFlowThread):
(WebCore::RenderLayerBacking::positionOverflowControlsLayers):
(WebCore::RenderLayerBacking::computeTransformOrigin):
(WebCore::RenderLayerBacking::computePerspectiveOrigin):
* rendering/RenderLayerBacking.h:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::requiresOwnBackingStore):
* rendering/RenderLayerCompositor.h:
* rendering/RenderMultiColumnSet.cpp:
(WebCore::RenderMultiColumnSet::adjustRegionBoundsFromFlowThreadPortionRect):
* rendering/RenderMultiColumnSet.h:
* rendering/RenderRegion.cpp:
(WebCore::RenderRegion::adjustRegionBoundsFromFlowThreadPortionRect):
* rendering/RenderRegion.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@164415 268f45cc-cd09-0410-ab3c-d52691b4dbfc