csscornervalue: Accept other values if x == y

csscornervalue: Accept other values if x == y

Most corners are square, so x == y. In that case, just accept either of
them. This makes the corner value unnecessary.

In fact none of the corner values in the widget-factory are needed, so
this spares us around 500 corner value allocations.

css value stats before:

GtkCssBgSizeValue: 23
GtkCssIdentValue: 25
GtkCssPositionValue: 81
GtkCssCornerValue: 556
GtkCssArrayValue: 143
GtkCssStringValue: 33
GtkCssPaletteValue: 29
GtkCssImageValue: 2765
GtkCssColorValue: 1452
GtkCssFilterValue: 3
GtkCssRgbaValue: 1092
GtkCssShadowValue: 708
GtkCssEaseValue: 33
GtkCssBorderValue: 2
GtkCssTransformValue: 11
GtkCssDimensionValue: 882
GtkCssShadowsValue: 584
SUM: 8428

and after:

GtkCssColorValue: 1452
GtkCssFilterValue: 3
GtkCssRgbaValue: 1092
GtkCssShadowValue: 708
GtkCssEaseValue: 33
GtkCssBorderValue: 2
GtkCssTransformValue: 11
GtkCssDimensionValue: 882
GtkCssShadowsValue: 584
GtkCssBgSizeValue: 23
GtkCssIdentValue: 25
GtkCssPositionValue: 81
GtkCssArrayValue: 143
GtkCssStringValue: 33
GtkCssPaletteValue: 29
GtkCssImageValue: 2765
SUM: 7872

8428 to 7872 is a 556 reduction (6.5%)

asdf