GUI: UIGridLayout elements not centered

[eruvanos]

Bug Report

Creating a UIGridLayout with one column or one row plus spacing, the widget is not placed centered.

System Info

Run this and paste the output here: python -m arcade

Actual behavior:

UIGridLayout applies the horizontal/vertical spacing even when only one row/column exists

Expected behavior:

No spacing for single row/column

Steps to reproduce/example code:

def test_widgets_are_centered(window):
    ui = UIManager()
    grid = UIGridLayout(row_count=1, column_count=1, horizontal_spacing=10, vertical_spacing=10)
    ui.add(grid)

    dummy1 = UIDummy(width=100, height=100)
    grid.add(dummy1, 0, 0)

    ui.execute_layout()

    assert dummy1.rect.bottom_left == Vec2(0, 0)