Set the uniform value to use when the vertex array is disabled

Property definitions

gamnit $ AttributeVec4 :: uniform
	# Set the uniform value to use when the vertex array is disabled
	fun uniform(x, y, z, w: Float) do if is_active then glVertexAttrib4f(location, x, y, z, w)
lib/gamnit/programs.nit:137,2--138,91