frustum: make clipspace coords configurable

* now users can override clip space coords
* add more desc to header
* add call version for _corners_at

include/cglm/call/frustum.h

@@ -29,6 +29,12 @@ CGLM_EXPORT
 void
 glmc_frustum_box(vec4 corners[8], mat4 m, vec3 box[2]);
 
+CGLM_EXPORT
+void
+glmc_frustum_corners_at(vec4  corners[8],
+                        float splitDist,
+                        float farDist,
+                        vec4  planeCorners[4]);
 #ifdef __cplusplus
 }
 #endif