Add googletest by git submodule.

.gitmodules

 [submodule "external/theft"]
 	path = external/theft
 	url = https://github.com/silentbicycle/theft.git
+[submodule "external/googletest"]
+	path = external/googletest
+	url = https://github.com/google/googletest.git

WORKSPACE.bazel

 workspace(name = "IMGRender")
 
 # Google test for unittest purposes
-load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
-git_repository(
+local_repository(
     name = "gtest",
-    remote = "https://github.com/google/googletest",
-    branch = "v1.10.x",
+    path = "./external/googletest"
 )
 
 # Theft for property-based testing

googletest @ 58d77fa8

+Subproject commit 58d77fa8070e8cec2dc1ed015d66b454c8d78850