carpet: add dual scroll animation effect

[okcomput3]

Add new scroll animation modes that simulate an ancient scroll unrolling
from both sides toward the middle.

Two new direction options:

• direction 4: horizontal scroll (left and right edges roll toward center)
• direction 5: vertical scroll (top and bottom edges roll toward center)

The effect creates two synchronized rolls that start at opposite edges
and meet in the middle when opening, and roll back outward from the
center when closing.

[soreau]

It looks decent at a glance and judging from your video. However we need to take special care in the shaders regarding whitespace, since uncrustify doesn't enforce anything there. Also, the metadata xml should be fixed to make it more obvious what these new options do.