Selected handles always use the same color as the outline. Previously
they were almost always using it, except while actively transforming -
the outline was brighter but the handle itself was not.
With this, there's no need for handle colors state for the shader,
it is enough to put in flags which handles are selected vs which
ones are merely drawn.
Current look of VSE timeline view strip transformation handles makes
them somewhat "too narrow", especially after recent changes that
made them more narrow than before (handle tweaking feature) and
a strip visual change that made strip outline not go outside of strip
bounds. They are now just 2px wide, effectively.
This changes their look as outlined in #123332 design task:
- The inset dark line is no longer over the handles, but rather
"inside" of them (except when handles are semitransparent, i.e.
for strips that are not selected).
- The handles themselves have rounded corners.
Now the strip outline (1 point for unselected strips, 2 point outline +
1 pt dark inset) takes monitor DPI / user preference line width into
account, via the usual U.pixelsize machinery.
