Re-implement gradient selections

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.

src/sp_lev.c

@@ -4683,6 +4683,8 @@ long x, y, x2, y2, gtyp, mind, maxd, limit;
 
     switch (gtyp) {
     default:
+        impossible("Unrecognized gradient type! Defaulting to radial...");
+        /* FALLTHRU */
     case SEL_GRADIENT_RADIAL: {
         for (dx = 0; dx < COLNO; dx++)
             for (dy = 0; dy < ROWNO; dy++) {