new project function for projecting Z

someitmes we need to project z only. this function reduces a few calculations and parameters.

include/cglm/call/project.h

@@ -25,6 +25,10 @@ CGLM_EXPORT
 void
 glmc_project(vec3 pos, mat4 m, vec4 vp, vec3 dest);
 
+CGLM_EXPORT
+float
+glmc_project_z(vec3 pos, mat4 m);
+
 CGLM_EXPORT
 void
 glmc_pickmatrix(vec2 center, vec2 size, vec4 vp, mat4 dest);