Destroy buffer_array and buffer_element

Property definitions

gamnit $ SpriteContext :: destroy
	# Destroy `buffer_array` and `buffer_element`
	fun destroy
	do
		glDeleteBuffers([buffer_array, buffer_element])
		assert glGetError == gl_NO_ERROR

		buffer_array = -1
		buffer_element = -1
	end
lib/gamnit/flat/flat_core.nit:1130,2--1138,4