Bug 244966

Whenever a SVG document fragment is placed in an enclosing HTML/CSS context, it receives its position per CSS rules (depending on 'display', 'position', etc.). These are potentially sub-pixel positions, such as x=8, y=79.8456. For HTML/CSS renderers the final position (in local coordinates) is pixel snapped, to enforce device-pixel alignment (no off-device-pixel painting). In WebCore "adjustedPaintOffset" (== paintOffset [external] + location() [intrinsic]) is the quantity that's pixel snapped within the Render*::paint() methods. The SVG render tree does not pixel snap any coordinates/lengths, except for the outermost <svg> renderer. There should be no visual difference between a HTML document drawing a box with a green background, and an equivalent SVG document that does the same using a <rect> element. Therefore the legacy SVG engine always rounded the 'adjustedPaintOffset' to the nearest integer, simulating the effect in the HTML/CSS rendering tree (not correct in all deviceScaleFactor configurations though!). In LBSE, pixel-snapping or rounding the 'adjustedPaintOffset' in RenderSVGRoot won't have the same affect as in the legacy engine: LBSE uses self-painting layers to paint the SVG document -- therefore any transformation needs to be applied on layer-level to affect the descendant renderers (that themselves use self-painting layers!). RenderLayer::paintLayerByApplingTransform() already contains all the code necessary to place outer <svg> elements on device-pixel aligned boundaries. The subpixelOffset is applied as transformation once for the outermost <svg>, and ignored (set to zero) for any other descendant SVG renderer. Pixel snapping is only relevant from RenderView downwards to the (and including) RenderSVGRoot -- the rest of the SVG subtree is not using pixel snapping / device pixel alignment. This will allow us to enable testing the few hundred tests in svg/dynamic-updates, that are currently skipped in LBSE (because they all fail). All of them showed subtle rendering differences caused by the currently _missing_ device pixel alignment for RenderSVGRoot.