Work around E3M2 pillar collision issue

Disable SV_TryUnstick when using pr_checkextension.

Fixes Shpoike/Quakespasm#50.

Quake/sv_phys.c

@@ -905,8 +905,9 @@ void SV_WalkMove (edict_t *ent)
 	clip = SV_FlyMove (ent, host_frametime, &steptrace);
 
 	// check for stuckness, possibly due to the limited precision of floats
-	// in the clipping hulls
-	if (clip)
+	// in the clipping hulls. Disable when using pr_checkextension to avoid
+	// https://github.com/Shpoike/Quakespasm/issues/50.
+	if (clip && !pr_checkextension.value)
 	{
 		if (fabs (oldorg[1] - ent->v.origin[1]) < 0.03125 && fabs (oldorg[0] - ent->v.origin[0]) < 0.03125)
 		{ // stepping up didn't make any progress