WD-specific fixes made. Note that some of these won't show up until we
actually switch over to WD.
Edits to 4.5 Animatable properties

--HG--
extra : rebase_source : 53a8a5d48ec15ec98fd194a87132c3bbfb478eed
Replace [web-anim] with [web-animations]
Remove orphaned reference to merge composition operation
Edits to 4.6 Animation of property types
Fix links to 'associated with a timeline'
Fix bibliography references for CSS transitions and transforms
Tidy up box formatting
Fix procedure for evenly distributing keyframes

Steps 2 and 3 of the existing algorithm ensure that offsets are specified for
the (perhaps temporary) first and last keyframes only if there are no keyframes
present with offsets of 0 and 1 respectively. This means that if keyframes with
offsets of 0 and 1 are present, but the first or last keyframes lack specified
offsets, step 4 will fail.

This patch fixes this bug by always ensuring that the first and last keyframes
have specified offsets, so that step 4 can assign offsets to any interior
keyframes with unspecified offsets.

This patch also removes the use of temporary keyframes. These are not required,
as if the last first or last keyframes already have specified offsets, no
interpolation to calculate offsets is required.