Add SwiftShader submodule.
Related to #9
diff --git a/.gitmodules b/.gitmodules
index a60c692..2524924 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -42,3 +42,6 @@
path = third_party/pybind11
url = https://github.com/pybind/pybind11.git
branch = stable
+[submodule "third_party/swiftshader"]
+ path = third_party/swiftshader
+ url = https://github.com/google/swiftshader.git
diff --git a/third_party/swiftshader b/third_party/swiftshader
new file mode 160000
index 0000000..6652f0b
--- /dev/null
+++ b/third_party/swiftshader
@@ -0,0 +1 @@
+Subproject commit 6652f0b6428777b5a4a3d191cc30d8b31366b999
