sokol: add sg_isvalid wrapper to gfx (#12081)

vlib/sokol/gfx/gfx.c.v

@@ -18,6 +18,11 @@ pub fn shutdown() {
 	C.sg_shutdown()
 }
 
+[inline]
+pub fn is_valid() bool {
+	return C.sg_isvalid()
+}
+
 [inline]
 pub fn reset_state_cache() {
 	C.sg_reset_state_cache()

vlib/sokol/gfx/gfx_funcs.c.v

@@ -3,6 +3,7 @@ module gfx
 // setup and misc functions
 fn C.sg_setup(desc &C.sg_desc)
 fn C.sg_shutdown()
+fn C.sg_isvalid() bool
 fn C.sg_reset_state_cache()
 
 // resource creation, destruction and updating