This also adds a -t flag to the easy build script for convenience.
SwiftShader on Vulkan can now be used as follows.
git clone https://github.com/google/swiftshader.git
cd swiftshader/build
cmake .. && make -j && export SWIFTSHADER_LD_LIBRARY_PATH=`pwd`
cd ../filament
./build.sh -t debug && ./out/cmake-debug/samples/gltf_viewer -a vulkan
Note that third_party/swiftshader does not need to exist since we
already have Vulkan headers. I will remove it in a subsequent PR.
2f294549a7