This patch makes outward curves use a sliver of toolbox background-image
to mask some artifacts that are results of several semi-transparent
surfaces. This also changes the curve svg image a bit to makes
curve-border transitions more smooth.
With positioninnedtabs, the left-most tab might end up outside of the
window under some circumstances (for example when menubar is enabled).
This patch moves tabbrowser-tabs away from the window edge equal to the
curve-size and makes the pre-tabs spacer a bit narrower to make the
space not look too large when the spacer is shown.
This patch also adds some comments for svg.context-properties.content.enabled
which is a requirement as well as make default theme have separate
background-color for curves.
