Uncomments and makes available selection.gradient in Lua. (The backend C
code for this still existed, it just wasn't used.)
The only valid way to specify a gradient is with a table. I considered
adding non-table versions, but decided that there are too many
independent variables that can be optional. A non-table version, without
named parameters, would be confusing to read, especially since most of
the arguments are ints.
Also adds an impossible in the (possibly unreachable) case that
selection_do_gradient gets called with a bad gradient type.
copperwater
0befdbfd02