If there are more than 7 color stops, we can split the gradient into
multiple gradients with color stops like so:
0, 1, 2, 3, 4, 5, transparent
transparent, 6, 7, 8, 9, 10, transparent
...
transparent, n-2, n-1, n
and use the new BLEND_ADD to draw them on top of each other.
Adapt the testcae that tests this to use colors that work with the fancy
algorithm we use now, so that BLEND_ADD and transitions to transparent
do not cause issues.
The shader can only deal with up to 7 color stops - but that's good
enough for the real world.
Plus, we have the uber shader.
And if that fails, we can still fall back to Cairo.
The code also doesn't handle repeating linear gradients yet.
