[Extern/stb] Added stb_image 2.29 as an external

- Only stb_image (image format loading) has been added for now; stb_image_write and probably stb_truetype will be added later
  - It supports JPEG, PNG, TGA, BMP, PSD, GIF, HDR, PIC, PPM & PGM

extern/CMakeLists.txt

@@ -32,3 +32,4 @@ if (RAZ_USE_LUA)
 endif ()
 
 include(fastgltf)
+include(stb)

extern/stb.cmake

@@ -0,0 +1,13 @@
+#######
+# stb #
+#######
+
+project(stb)
+
+add_library(stb INTERFACE)
+
+file(GLOB STB_FILES stb/*.h)
+
+target_sources(stb INTERFACE ${STB_FILES})
+
+target_include_directories(stb SYSTEM INTERFACE stb)

extern/stb/LICENSE.txt

@@ -0,0 +1,16 @@
+Copyright (c) 2017 Sean Barrett
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.