cursor: add option to warp on output change

On output change, if the cursor is not already on the newly focused
output, it will now be warped to its center. The check is necessary,
since focusing outputs with the pointer will be implemented in
the future.

river/command/config.zig

@@ -21,6 +21,7 @@ const server = &@import("../main.zig").server;
 
 const Error = @import("../command.zig").Error;
 const Seat = @import("../Seat.zig");
+const Config = @import("../Config.zig");
 
 pub fn borderWidth(
     allocator: *std.mem.Allocator,
@@ -81,6 +82,18 @@ pub fn borderColorUnfocused(
     while (it) |node| : (it = node.next) node.data.damage.addWhole();
 }
 
+pub fn setCursorWarp(
+    allocator: *std.mem.Allocator,
+    seat: *Seat,
+    args: []const []const u8,
+    out: *?[]const u8,
+) Error!void {
+    if (args.len < 2) return Error.NotEnoughArguments;
+    if (args.len > 2) return Error.TooManyArguments;
+    server.config.warp_cursor = std.meta.stringToEnum(Config.WarpCursorMode, args[1]) orelse
+        return Error.UnknownOption;
+}
+
 /// Parse a color in the format #RRGGBB or #RRGGBBAA
 fn parseRgba(string: []const u8) ![4]f32 {
     if (string[0] != '#' or (string.len != 7 and string.len != 9)) return error.InvalidRgba;