Condense GLX extension lookup

Put them all into one function, and move the function pointers out of
glx_session_t, making them global variables (because them don't change
after initialized). Remove the function pointer typedefs and replace
them with the ones in glxext.h

We also only lookup the functions once per a lifetime of compton.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/render.c

@@ -1121,6 +1121,9 @@ static bool init_alpha_picts(session_t *ps) {
 
 bool init_render(session_t *ps) {
 	// Initialize OpenGL as early as possible
+#ifdef CONFIG_OPENGL
+	glxext_init(ps->dpy, ps->scr);
+#endif
 	if (bkend_use_glx(ps)) {
 #ifdef CONFIG_OPENGL
 		if (!glx_init(ps, true))