Move stb_image_write into its own translation unit

Rather than having the entire implementation within `emulator.cpp`,
causing incremental builds to be much slower, give it its own
translation unit `stb_image_write.c`.

CMakeLists.txt

@@ -92,7 +92,7 @@ endif()
 
 set(SOURCE_FILES src/main.cpp src/emulator.cpp src/io_file.cpp src/gl_state.cpp src/config.cpp
                  src/core/CPU/cpu_dynarmic.cpp src/core/CPU/dynarmic_cycles.cpp src/core/memory.cpp
-                 src/httpserver.cpp
+                 src/httpserver.cpp src/stb_image_write.c
 )
 set(CRYPTO_SOURCE_FILES src/core/crypto/aes_engine.cpp)
 set(KERNEL_SOURCE_FILES src/core/kernel/kernel.cpp src/core/kernel/resource_limits.cpp