[material] fix _set_alpha_test_function: missing a glEnable(GL_ALPHA_TEST)

[material] fix _set_alpha_test_function: missing a glEnable(GL_ALPHA_TEST)

Since the default alpha test function of GL_ALWAYS is equivalent to
GL_ALPHA_TEST being disabled we don't need to worry about Enabling/Disabling
it when flushing material state, instead it's enough to leave it always
enabled.  We will assume that any driver worth its salt wont incur any
additional cost for glEnable (GL_ALPHA_TEST) + GL_ALWAYS vs
glDisable (GL_ALPHA_TEST).

This patch simply calls glEnable (GL_ALPHA_TEST) in cogl_create_context