SVGs are rendered with subpixel precision. As such it can happen that
paths are rendered with less than 1px width or height and that they can
have a bounding box thinner than 1px. Due to an optimization such paths
were ignored when painting because their bounding box was incorrectly
calculated to be empty.
As a result horizontal or vertical lines inside SVGs were missing if:
* The SVG is displayed at viewbox size but the lines are defined with
less than 1px.
* The SVG contians 1px-thin lines, but is displayed at a size smaller
than viewbox size.
To prevent this, the bounding box of the path is now enlarged to contain
all pixels that are partially affected.
This improves the quality of our font rendering, especially when
animations are involved. Relevant changes:
* Skia fonts have their subpixel flag set, which means that individual
glyphs are rendered at subpixel offsets causing glyph runs as a
whole to look better.
* Fragment offsets are no longer rounded to whole device pixels, and
instead the floating point offset is kept. This allows us to pass
through the floating point baseline position all the way to the Skia
calls, which already expected that to be a float position.
The `scrollable-contains-table.html` ref test needed different table
headings since they would slightly inflate the column size in the test
file, but not the reference.
CSS filters work similarly to canvas filters, so it makes sense to have
Gfx::Filter that can be used by both libraries in an analogous way
as Gfx::Color.
