fix: Fix ui.Group.SetMargined() implementation to convert to c_int

[jthat]

When porting the control gallery example (#14) I ran into the following build error:

% zig build
install
└─ install control-gallery
   └─ zig build-exe control-gallery Debug native 1 errors
src/ui.zig:688:31: error: expected type 'c_int', found 'bool'
        uiGroupSetMargined(g, margined);
                              ^~~~~~~~
src/ui.zig:678:59: note: parameter type declared here
    pub extern fn uiGroupSetMargined(g: *Group, margined: c_int) void;
                                                          ^~~~~

This change fixes ui.Group.SetMargined() to convert the bool correctly to a c_int for the call to uiGroupSetMargined().