Moves glm, GSL and libpng to 3rdparty

Moves glm and GSL to 3rdparty
Moves libpng to 3rdparty

.gitmodules

@@ -20,10 +20,10 @@
 	path = rsx_program_decompiler
 	url = https://github.com/RPCS3/rsx_program_decompiler
 [submodule "GSL"]
-	path = GSL
+	path = 3rdparty/GSL
 	url = https://github.com/Microsoft/GSL.git
 [submodule "libpng"]
-	path = libpng
+	path = 3rdparty/libpng
 	url = https://github.com/RPCS3/libpng
 [submodule "Vulkan/glslang"]
 	path = Vulkan/glslang

.travis.yml

@@ -53,7 +53,7 @@ before_install:
     fi;
 
 before_script:
- - git submodule update --init asmjit 3rdparty/ffmpeg rsx_program_decompiler GSL libpng Vulkan/glslang Vulkan/Vulkan-LoaderAndValidationLayers
+ - git submodule update --init asmjit 3rdparty/ffmpeg rsx_program_decompiler 3rdparty/GSL 3rdparty/libpng Vulkan/glslang Vulkan/Vulkan-LoaderAndValidationLayers
  - mkdir build
  - cd build
  - if [ "$TRAVIS_OS_NAME" = "linux" ]; then cmake ..; else cmake .. -DLLVM_DIR=/usr/local/opt/llvm36/lib/llvm-3.6/share/llvm/cmake; fi