LibGL: Implement glAlphaFunc()

This implements glAlphaFunc() for setting alpha test func and ref value
and also allows enabling and disabling GL_ALPHA_TEST

Userland/Libraries/LibGL/GLBlend.cpp

@@ -13,3 +13,8 @@ void glBlendFunc(GLenum sfactor, GLenum dfactor)
 {
     return g_gl_context->gl_blend_func(sfactor, dfactor);
 }
+
+void glAlphaFunc(GLenum func, GLclampf ref)
+{
+    return g_gl_context->gl_alpha_func(func, ref);
+}