Fixes issue that a viewport texture doesn't work with particle shader

drivers/gles3/rasterizer_storage_gles3.cpp

@@ -5895,7 +5895,10 @@ void RasterizerStorageGLES3::update_particles() {
 							tex = resources.white_tex;
 						} break;
 					}
+				} else if (t->proxy && t->proxy->tex_id) {
 
+					target = t->proxy->target;
+					tex = t->proxy->tex_id;
 				} else {
 
 					target = t->target;